MAC: System Prefs-->Sharing-->Internet-->Airport Options
I can't go further because I don't have an airport card...sorry. You could also check "Ethernet" off to share the connection via ethernet and then just plug in the mac.
I think you can also do something with the wireless via Airport Admin Utility
Heres a thread about networking. You more than likey need to do what the second poster said of internet sharing over airport. Any way i have not done the two but if you mix the two in you should be fine.