While in car, call the number shown on a radio display. Tell him you're activating pre-paid Mitsubishi subscription.
When asked, tune to channel #0 to get Sirius unit's serial number. Tell that number to the guy over the phone, and in next 2-3 minutes your radio is activated.

When your subscription is over, remove Sirius box out from the car, try to sell in on Ebay or just leave it in the garbage bin, as Sirius technology, with their so-so playlists and severely outdated slightly-better-than-FM quality (32 to 64 kbps) has no future.