I'm running in to the same problem with my skin editor to code it so that you can do horizontal side bar.
here's what I know:
The XML code is easy enough to modify outside of any editor (notepad,xml spy etc)
You can use one of the skin editors out there to "help you out" as no editor is yet feature rich and encompases all aspects of centrafuse take your pick.
I'll promote my own skin editor and I'm trying to build it up to be "feature rich" but I've got a long road to travel on that.
With my skin editor you can visually get the initial menu button positioning with it and edit the menu bar in photoshop. Then you all that is left is to manually edit this XML area:
Skin\Screens\Main\Menu\ <- Edit Width and Height to match your photoshop graphic and your good.
Here's a mock up of my skin using a horizontal menu bar...it's crappy but I did it in about 5 mins. *NOTE* buttons are only able to show text left to right so your buttons can't be tall and have the text align top to bottom.
Check out the Skins forum and search for my skin editor links V.20 is the latest.
P.S. the menu bar really doesn't slide it just displays really quickly, your eye tracking provides the slide effect
Any more questions let me know, I'll try and help you out.