The only software aside from google sketchup is a program called 'topescope which lets you view your completed files before you send them off and spend money on the prints.
Supported formats are stl, dxf gts, nff, q3o, obj, off, ply, tri, uo
I have also attached the ruby files needed for sketch (see next step). There are two of them and I will explain where they need to go.
Step 1: Add files to sketchup
The files should go right inside the Google SketchUp (your version) folder.
There are 3 files in the photo but you only need 2. One of them is another unrelated ruby file.