Modeltalker site- It seems to be for Windows , I haven't seen any mention of Mac for this.
http://www.modeltalker.com/


this has a listing of many programs and some for Mac
http://www.etriloquist.com/alslinks.html

http://www.joesalsadvocacy.com/pages/speech.htm#free

I'll place a copy this on to our ALS forum - they might have some better suggestions for you.