Hi Scott,
Normally you should use the external dependencies as you did. You may have to (re)set environment variable FGLLDPATH in order to make the external dependency (lib_main.42m) be found on rebuild, but the problem seems to appear at execution? Is your library compiled with the same FGL version as the other modules? Which version of FGL is it? Do you have the same problem when compiling, linking and running your application outside Genero Studio?
Regards,
Romain