First, you need to have remote command-line login (with the SSH protocol) enabled on other computer -- open System Preferences -> Sharing, and enable Remote Login (also, make sure the Allow access for: option enables the account you want to log into).
To make the connection, run Terminal, and use the command
ssh remoteuser@remoteBonjourName.local, where remoteBonjourName.local is the computer name of the other machine with most punctuation removed, spaces converted to "-", and ".local" added to the end (check its Sharing prefs, right under the Computer Name)
Alternately, you can use Terminal's New Remote Connection option (under the Shell menu), select Secure Shell (SSH) from the Service list, and it should show all Macs running SSH on the right side. Select the one you want to connect to, enter the user you want to log in as in the User field, and click Connect.