In the installation tree, you should find a n executable in lib/gimp/2.0/plug-ins/file-ps/file-ps (could have an extension if OSX requires it).
If you use ldd lib/gimp/2.0/plug-ins/file-ps/file-ps it will list everything that the plug-in requires to run (and where it finds it).
Now, if you are missing a library you get a more ominous message: Plug-in crashed: "file-ps"
(/path/to/your/lib/gimp/2.0/plug-ins/file-ps/file-ps) as I just experienced (installed GS version incompatible with the one my Gimp was compiled for).
Given your error message the GS code runs but doesn't understand the file contents, so IMHO your problem is not with your GS installation but with the EPS file. Can you attach it here? Did you try to open other files?
If you use ldd lib/gimp/2.0/plug-ins/file-ps/file-ps it will list everything that the plug-in requires to run (and where it finds it).
Now, if you are missing a library you get a more ominous message: Plug-in crashed: "file-ps"
(/path/to/your/lib/gimp/2.0/plug-ins/file-ps/file-ps) as I just experienced (installed GS version incompatible with the one my Gimp was compiled for).
Given your error message the GS code runs but doesn't understand the file contents, so IMHO your problem is not with your GS installation but with the EPS file. Can you attach it here? Did you try to open other files?