Set up OSX to act as router via Internet Sharing
You need to enable routing.  I'm pretty sure on OS X this is a sysctl(like FreeBSD):

http://securityreliks.securegossip.com/2010/10/enabling-ip-forwarding-on-mac-osx/

doing:

# sysctl -w net.inet.ip.forwarding=1


as root should enable this.