I just finished my reprap mendel and was able to load and print the test.stl file, but then I went and downloaded other files like the bed cooling fan mod and the mendel stl for all the pieces and whenever I try to load one of the files, it doesn't show up in the window where the picture of the part should be and print isn't highlighted. Im a total noob, of coarse! Thanks in advance, oh I'm using Ubuntu Linux if that matters.

This is a really stupid thing I can't beleive nobody can help me, if you even have a stupid idea I'd like to hear it!