What exactly do you mean?
Assuming you have defined the tag in the tag database, check the box on the tag database editor next to the 'Alarm' button. The alarm configuation editor should pop up. Set the alarm conditions, say okay, and done.
If the SE Server is running (and you are running CPR7 PLUS of SE), you need to issue the 'AlarmAcceptEdits' command from a command prompt. For earlier versions, from CPR 7 and below, shut down and restart the alarm handler from the command line.