Invisible Models?

To be specific, I’m having trouble with two of the models in this K-On Model pack, the Fender Mustang guitars:
http://forum.facepunch.com/showthread.php?t=1118985

When spawned, this error occurs:
[gamemodes\sandbox\entities\effects\propspawn\init.lua:21] Model missing: models/k-on/mustang.mdl

The models do not have proper spawn icons in their spawn lists, but when spawned, both the strapped and unstrapped version of the guitars seem to spawn, but are invisible. When ignited they both appear vaguely guitar shaped. Messing with the material and color tools do not work. You can seem to grab on to them and manipulate it with the physgun, but again, the models themselves cannot be seen.

This same error happens to me and two friends, and we are all confounded on how to fix it as the guitars were essentially all we wanted out of the pack. We are further confused by the fact that the models show up fine in the screenshots in the actual thread, but refuse to work for us in game and there seems to be no mention of the error in the thread itself. Did we perhaps install it incorrectly? We are all currently using the 0.4a version, but have tried it and gotten the same results with .3a.

Any help would be greatly appreciated, thanks.

It is more than likely you have not installed it correctly. Same thing happened to me, then I installed the lua, models, materials etc. into my garrysmod/garrysmod