Hi Peter,
Attached is a zip file with all the Creo models for the challenge. Included in the zip is a DXF file (2013_s4s_chassis2cut.dxf) for the inline chassis which is the one featured in the tutorials. The inline rear axle has a straight knurl in the centre of the rear axle and you use the white plastic gears..
If you purchased your components packs in the last year or so, you may have been supplied with rear axles for sidewinder mounted motor so I have also attached the zipped files for that chassis. The sidewinder dxf is - 2014_sidewinder_chassis2cut.dxf. This rear axle has no centre knurl, the knurl on one end is longer than the other for the larger black pinion with the small pinion going on the motor.
Illustrations of the two arrangements can be seen here: http://communities.ptc.com/docs/DOC-4867