In your exo-terminal-emulator.desktop file, change:

Categories=Utility

...to read:

Categories=X-XFCE;X-Xfce-Toplevel;Utility;

. . . and it was there in an instant!!!! Thanks a lot fro your help!

...to read:

Categories=X-XFCE;X-Xfce-Toplevel;Utility;

WebBrowser
MailReader
FileManager

But not:

TerminalEmulator

And i don't understand why.

The content of the menu file is:

<!DOCTYPE Menu
  PUBLIC '-//freedesktop//DTD Menu 1.0//EN'
  'http://standards.freedesktop.org/menu-spec/menu-1.0.dtd'>
<Menu>
    <Name>Xfce</Name>
    <MergeFile type="parent">/etc/xdg/menus/xfce-applications.menu</MergeFile>
    <Menu>
        <Name>Development</Name>
        <DirectoryDir>/home/< . . . >/.local/share/desktop-directories</DirectoryDir>
        <Include>
            <Filename>python2.6.desktop</Filename>
        </Include>
        <AppDir>/home/< . . . >/.local/share/applications</AppDir>
        <Include>
            <Filename>python2.7.desktop</Filename>
        </Include>
    </Menu>
    <Menu>
        <Name>Games</Name>
        <DirectoryDir>/home/< . . . >/.local/share/desktop-directories</DirectoryDir>
    </Menu>
    <Menu>
        <Name>Education</Name>
        <Deleted/>
    </Menu>
    <Layout>
        <Merge type="menus"/>
        <Separator/>
                <Filename>exo-web-browser.desktop</Filename>
                <Filename>exo-mail-reader.desktop</Filename>
                <Filename>exo-file-manager.desktop</Filename>
                <Filename>exo-terminal-emulator.desktop</Filename>
        <Separator/>
        <Menuname>Settings</Menuname>
        <Separator/>
        <Menuname>System</Menuname>
        <Menuname>Accessories</Menuname>
        <Menuname>Other</Menuname>
        <Menuname>Development</Menuname>
        <Separator/>
        <Menuname>Office</Menuname>
        <Menuname>Network</Menuname>
        <Menuname>Graphics</Menuname>
        <Menuname>Multimedia</Menuname>
        <Menuname>Games</Menuname>
        <Separator/>
        <Merge type="files"/>
    </Layout>
    <Menu>
        <Name>Other</Name>
        <Include>
            <Filename>panel-desktop-handler.desktop</Filename>
        </Include>
        <AppDir>/home/< . . . >/.local/share/applications</AppDir>
        <Include>
            <Filename>openjdk-6-java.desktop</Filename>
        </Include>
        <Include>
            <Filename>openjdk-7-java.desktop</Filename>
        </Include>
    </Menu>
    <Menu>
        <Name>Network</Name>
        <Include>
            <Filename>icedtea-netx-javaws.desktop</Filename>
        </Include>
        <AppDir>/home/< . . . >/.local/share/applications</AppDir>
    </Menu>
    <Menu>
        <Name>System</Name>
        <Include>
            <Filename>manage-print-jobs.desktop</Filename>
        </Include>
        <AppDir>/home/< . . . >/.local/share/applications</AppDir>
        <Exclude>
            <Filename>debian-uxterm.desktop</Filename>
        </Exclude>
        <Exclude>
            <Filename>debian-xterm.desktop</Filename>
        </Exclude>
        <Include>
            <Filename>hplj1020.desktop</Filename>
        </Include>
        <Include>
            <Filename>gnome-power-statistics.desktop</Filename>
        </Include>
    </Menu>
    <Menu>
        <Name>Multimedia</Name>
        <Include>
            <Filename>gnash.desktop</Filename>
        </Include>
        <AppDir>/home/< . . . >/.local/share/applications</AppDir>
    </Menu>
</Menu>

And exo-terminal-emulator.desktop looks like this:

[Desktop Entry]
Version=1.0
Type=Application
Exec=exo-open --launch TerminalEmulator
Icon=/usr/share/icons/moblin/48x48/apps/moblin-utilities-terminal.png
StartupNotify=false
Terminal=false
Categories=Utility
OnlyShowIn=XFCE;
Name=Terminal Emulator
Name[ar]=محاكي الطرفية
Name[ast]=Emulador de terminal
Name[be]=Эмулятар тэрмінала
Name[bn]=টার্মিনাল ইম্যুলেটর
Name[ca]=Emulador de terminal
Name[cs]=Emulátor terminálu
Name[cy]=Efelychydd Terfynell
Name[da]=Terminal-emulering
Name[de]=Terminal
Name[dz]=ཊལ་མི་ནཱལ་ནུས་འཕྲུལ།
Name[el]=Εξομοιωτής τερματικού
Name[en_GB]=Terminal Emulator
Name[es]=Emulador de terminal
Name[et]=Terminaliemulaator
Name[eu]=Terminal emuladorea
Name[fi]=Pääteohjelma
Name[fr]=Émulateur de Terminal
Name[gl]=Emulador de terminal
Name[he]=מסוף
Name[hr]=Emulator terminala
Name[hu]=Terminálemulátor
Name[id]=Emulator Terminal
Name[it]=Emulatore di terminale
Name[ja]=ターミナルエミュレータ
Name[kk]=Терминал эмуляторы

I tried different options in Preferred Applications but no go . . .

TIA for any help.

In Xubuntu, the default menu is actually located at: /etc/xdg/xdg-xubuntu/menus/xfce-applications.menu.

However, if ~/.config/menus/xfce-applications.menu exists, it overrides the default one. Make sure you edit this file as your user account, not root.

If still no love, perhaps you can post the contents of the menu file?

Ok, thanks! I'll try that way and will get back then. Oh, perhaps i should say: It's on a Powerbook G4 running Debian wheezy with Xfce 4.8 i'm having this problem.

However, if ~/.config/menus/xfce-applications.menu exists, it overrides the default one. Make sure you edit this file as your user account, not root.

If still no love, perhaps you can post the contents of the menu file?

a) /etc/xdg/menus/xfce-applications.menu

b) ~/.config/menus/xfce-applications.menu

Which one of them ist relevant and which one not? Seems to me, as far as i understand that b) refers/integrates a) . . . (???)

Anyway, my problem is: under the xfce button there appear on top 2 seperators wit nothing inbetween (i'd like to have there like it is in my netbook running xubuntu 11.10 Browser, Mailreader, Filemanager, Terminal). Then there come Preferences, Separator, System, Accessories ecc. ecc. I edited both the files (as root) to have between the 2 separators that programs but nothing changed. What did i wrong?

Sorry in advance for the probably very dumb question