Generally what it means when it says ready is that you can plug a tuner into the back of it. It doesn't already have the tuner inside most likey.
From Crutchfield"Sirius Satellite Radio controls (Sirius Radio subscription, tuner, and antenna required to receive the satellite radio signal) service is available only in the lower 48 states, not available in Alaska, Hawaii, or U.S. territories
Edit:Sorry my XM actually have two inputs for antenna.