Okay, I just looked at the manual for your USR8000. A trigger is entered as a port number. When your computer makes a connection to the trigger port number, it activates the incoming ports for that computer. So it looks like you should enter the outgoing port number for MSN (which, I think, is 1863) as the trigger and enter 6891 as the incoming ports in association with that trigger.

As to why it doesn't appear to be saving, I can't tell you. I've never used one of these boxes.