I haven't looked at GNU Solfege for years, will check it out again.
@SteveO - I'll take a guess and say that the default version of java on your mac is 1.5, whereas it needs to be 1.6 or higher. I don't have a mac, but I know there is a Java Preferences utility/application somewhere where you can specify the default version. Let me know if you come right, I'd like to able to tell the other guys who're having trouble.