Turn on the Personal File Sharing on one of the Macs, then from the other one Select the Connect to Server... from the Go menu in the Finder.
Once the window comes up, type "afp://<name of other Mac>.local" and click on the Connect button.
e.g. If your iMac is called "My-iMac" you need to type "afp://My-iMac.local"
You will then be prompted for a valid username and password on the other computer. Enter them and you're there.
An icon for the other computer should also appear in the Network section of the Finder, but I've found that the above method is more reliable.