As far as I know all that XM radio ready means is that the head unit (radio) is compatible with xm plug and play tuner. What this means is that to get xm sat radio you need to buy and install the tuner and an xm antena and your done.
You night actally have the tuner installed and just need a subscription like the previous poster said. If thats the case just call xm and as Hummer Guy said "they'll get you goin".
To figure out if you have the xm tuner installed try pressing the source button on the radio to see if it will change to XM1 or XM2. If it does then go to channel 0 or channel 1, i am not really sure, and your radio ID will display on the amber lcd on the front of the radio. This means you have the tuner and all you need is a subscription.
By the way if you got the H3 new and it does have the XM tuner, the dealer should have activated it for you with a free 1 or 3 month subscription! You should look into that. It gives you enough time to try the programing and see if you would like to stick with it.
Sorry for the long write up.
Hope I was of help