When I try to use any of the features in the Gaussian menu, I get an error message that the shared library I1.exe cannot be found.
The Gaussian menu in Chem3D was designed to interface with a Gaussian Mac application well before MacOS X that was never released by Gaussian. Gaussian has released a version of Gaussian for MacOSX (Gaussian98M). Chem3D was not designed for this version of Gaussian and will not directly interface to it. If you do have Gaussian 98M, then you can use Chem3D to create input files that you can run with G98M. You can also use G98M to create cube files for visualization of surfaces in Chem3D.