Poser 4 generates the geometry .rsr file automatically, if none is found. So even though you didn't download and install an geometry .rsr, you might have a corrupted .rsr now ....

Pro Pack and Poser 5 don't create or use geometry .rsr files, incidentally.

It's also possible that you've installed the .obj in the wrong location. When Poser can't find the geometry, you'll get several error messages, then the figure will finish loading, and even though it appears in the selection menu there's nothing visible onscreen.