http://wowace.com/wiki/Skinner
You don't need to add anything to it at all. The ONLY thing you need to do is move files from the AddonSkins folder to the SkinMe folder for whichever addons you have installed that you want skinned. All of the options are in-game for the viewport, which frames to have enabled, if you want skins turned off for certain frames, etc. Oh yeah - and if you want FuBar menus and tooltips skinned, you need to put the Tablet and Dewdrop skins into the SkinMe folder.