Sirius radio id???

Help!!! We need to activate the sirius in the wife's (new to us) 2008 Mini. She called sirius to add it to our account, but they need the radio sid #, which we don't know how to get (on our other sirius radios you just hit a button and it appears). We called the dealer and he said we have to call sirius to get the number. So which is it? My guess is that the dealer has to retrieve it from the car's computer or key??? Any help here would be appreciated. Thanks