I think you're better off buying a real TTY machine because there's no "good" TTY programs for Mac. You could try Nextalk on
NXi Communications Inc. - Home Page (2001-2006), tho you might need a intel Mac to run a Windows program WITHOUT having Windows on your system. There's a software that makes it possible, I believe. I forget the name of the software that does it. You could also try "Boot Camp" which will make intel Mac run Windows XP or whatever.
I do not know if you have a intel based Mac or a old PowerPC Mac, which I have a PPC Mac, too.
You can find used TTY machines less than what softty charges for the program. If you're deaf, you could rent a TTY from the state.