You are required to register here:
http://fourdeltaone.net/forum/ucp.php?mode=register
Then use your account information to log into the game.
Note:
you can either play alone (with Bots), or with your friends. Once they are
registered, you can add them to your friend list here:
http://fourdeltaone.net/friends
how to run zombie mode :
Direct Link
Torrent