I can compile a hat using SDK itemtest just fine, but when I go into TF2, go to itemtest there, and try to open the .mdl file for the hat it does not open the file. I select the .mdl, click Open, the file browser closes but the itemtest menu still shows that no .mdl is selected. I've chosen the correct .mdl and have not altered anything with it.

Anybody experienced this problem before? Or does anybody know what the problem is?