I looked into it for my SE Sport with JBL system. Mine had a button for it, but when I called to activate it they told me I do not have the receiver, and would need to purchase the unit that mounts behind the radio. I ended up deciding that it's not worth the cost. Once I install an aftermarket audio/video/navigation head unit I will get Satellite Radio.
If you have the Satellite Radio receiver installed already, you basically just need to call XM or Sirius and tell them your VIN and they can activate it for you. I believe you have to be in the car and follow a few steps. I think we determined that my radio didn't have the receiver because it did nothing when I hit the "SAT" button. If you have the receiver I believe it's supposed to show some ID # or code they use to activate it. That's all the help I can give from memory
Hope it helps.