Google Sketchup 6
Currently the Colladae exporter of Google Sketchup is the only one compatible with the XPacker from Bobs Track Builder.
Google Sketchup 7
Sketchup 7.1 is available as a free download. It does have a Colladea exporter but is not compatible with XPacker. It handles the graphics manipulation better than version 6 and has less of a tendency to crash. Not all of the textures are compatible with BTB and you will benefit most by making your own set of textures. This is best done with either Photoshop, Paintshop or something similar.