Adding textures is simple (in Sketchup 6 - I use this one). Select some face you want to add a texture. The faces must appear as white/gray (not blue - if so, right click on "blue" face and choose "Reverse Faces").
Go to menu: File - Import - and browse your PC for .png, .jpg, .psd (unfortunately it does not support .dds, but not very big problem). Choose option "use as texture" before proceding. When you hit "Open", the image will be glued to your mouse pointer. Attach it to the face (dragging till whole face is covered). Voila
Then you can right-click on the textured face and go to "Position" - scale, turn, etc.
Adding dds instead of jpg's. After you export to Google Earth 4 (.kmz)... I guess you have read a tutorial by Biggles1212 entitled "Making Objects For BTB - full Sketchup tutorial"... If not, I strongly recommend it to you, great!
http://www.racedepartment.com/bobs-track-builder/24910-full-tutorial-objects-for-btb-sketchup.html
So, after export, change .kmz to .zip and unpacking the thing, open XPacker, add your object - the file with extension .dae (e.g. yourNewModel.dae) will be in folder "models".
Before that (or in the same time) - go to unpacked kmz/now zip open the folder "images", take the .jpg picture and convert it into .dds. You can name the file as you want (XPacker is quite wise and changes the paths without problems
Then add a new Texture to your xpack (the dds file you've just made), make a Material from it and replace Texture material from Sketchup (will be called probably "material0" or "texture0" - the file in xpacker's "Textures" folder will be "texture0.jpg" or similar) with yours. If you save changes, you can delete that skechup's .jpg texture, which is not needed anymore.
I guess I don't need to specify advantages of .dds files and disadvantages of jpg's or png's
You can of course use built-in textures from Sketchup - some of them are nice.
Back to Sketchup: after you add a texture from outside, it will be (in panel Materials [go to menu: Window-Materials] in "In Model" tab from the roll-over list. Then simply click the thumbnail and then click another face with the paint bucket. This is so simple
In that "In Model" you will see all colors/textures you actually see on the screen.
I recommend to put textures to all faces (from one side, you don't need to attach them to both sides of faces) and check it couple times, then after export you will not have textures like "blank_0_0_0.dds" or something like that. Ok, it's nothing too bad, but you have not to delete them from xpack after adding a new object - BTB probably will crash due to missing textures...
The whole process in quite easy, but keep your eye for possible mistakes. I hope you will like it as I do
When you are not sure, make a backup of "good" xpack and also make a backup of your project before overwriting existing xpack!