What does the Arduino ethernet shield connect to?
Number 2 is the ethernet port.



On the left is an Ethernet port and the right a Modem/Telephone port.



In order to connect with one wire you need to use a Cross-Over cables and set IP addresses in the same range for both devices! 192.168.x.x; 10.0.0.x .. etc.  (or set your mac as a DHCP server)



If you don't want to use the cross over system then you will need a router that has a DHCP server and assigns IP's to each device automatically. And then check the Router on its ARP table what the IP of your Arduino Ether shield is for communication.

If you have a newer airport then you can plug the Arduino Ethersield into any one of the free ports on the Right Side



If you have an old airport then you will need to get a switch and remove the Blue cable form your Imac. and plug it into the switch. Then get 2 new cables and connect your iMac to the switch and the Arduino EtherShield to your Switch.



Here are some setup examples