If you have local account on the system
When you see XDM prompt (little window with user/password) type your user name and password. You can use either your local password or your kerberos password . If you enter your local password you would get into the system, but you would not have AFS or Kerberos tickets. This option is useful only if there is problem with central computers and you need to get in anyway. If you enter your kerberos password you would get into the system and in addition you would be able to use AFS system and move around Fermilab without password, using your kerberos tickets. I strongly reccomend to use kerberos password . Use your local password as a reserve.
If you do not have local account on the system
You can use general ckm account to get into the system