Here is how to do it with an Xbox.
Replace "Xbox" with "Arduino".
edit: This has graphics to go with the description.
http://www.maclife.com/article/howtos/how_share_your_wireless_internet_connection_ethernet
It appears the Mac will assign 192.168.2.1/24 to the ethernet port. You will assign these to your Arduino:
ip 192.168.2.2
netmask 255.255.255.0
gateway 192.168.2.1
dns 192.168.2.1
...or use dhcp on the Arduino. It appears the Mac will set up a dhcp server on the ethernet connection for you. 
If you are looking to connect to the Arduino from the IPad, that will take a bit more work. You will need to port forward the wireless ip (192.168.0.x) to your Arduino localnet ip (192.168.2.2) in your Mac.