In the interest of self-promotion, here are some instructions for building XFCE-4.4.0 from source http://cblfs.cross-lfs.org/index.php/Xfce.  "All" of the dependencies (and instructions for building them) are cross-referenced as well.  The build instructions for Xfce-Panel include the optional patch to enable transparency.

That's not promotion, that's sharing! Very nice. I would have thought the requirements were steeper for building the panel, actually. I might take a stab at building the transparent panel myself :-)

Where is that folder located ?

From your question, I'll assume that you did not install from source ;-) This may be a bit of work, but you need to start by downloading the source for the panel from xfce.org. In order to build it, especially with the transparency patch, you will *also* need a dozen or more development libraries (xfce panel, gtk2, xorg, etc.) Not at all impossible, but when you get the source and patch it, then run ./configure, be prepared to do some detective work. It can be frustrating or fun, depending on how you look at it.

Go into you package manager and look up 'xfce'. Install every devel lib there is. Also libgtk2 devel. Probably also cairo-devel. That should at least get you started.

Good luck!

Could someone please explain how to apply the patch to make the panel transparent. I have seen the other posts that says to copy the patch to the 'sources' folder and do: patch -p0 < panel-cairo-20070125.patch. I do not know what the sources folder is, could someone explain this further, thanks.

I would imagine it means the folder which contains the source for xfce-panel. :-)