I assume you had the battery disconected for some time of the install?

My alarm had a bit of a funny the other day. Couldnt unlock the car using the blipper so I assumed the battery was dead. I unlocked the car just using the key in the door lock conveniently forgetting it wont disarm the alarm which of course went off when I opened the door....closed the door and hit the lock button and it worked first time and has done since.

I'm not that clued up on the Subaru alarm / immobiliser....does it need to 're-learn' the key if the battery has been disconnected? Maybe just try sticking the key in and turning the ignition on and off a few times?