If you've been given the key, there should be somewhere to put it.

Have you got an invoice for the alarm fitting in the service history, maybe you could ring them up and ask where it is?

Also - You sure it's the alarm that's at fault. Did you tocgh anything else, or short out anything by accident? The Mk2 had some daft oil pressure warning buzzer that sounds when the dash fucked up - which is sounds like your has.

Have you done simple stuff like checking fuses etc?