1.) remove the panel around the climate controls and stereo - this is actually much easier than it sounds (at least to me...tried doing that with all of my previous cars, to no avail). just unscrew the two screws below the controls (you might not see them unless you get down on the floor of the car), then gently pry around the trim that goes around the stereo and climate controls. there's two clip type of things near the top of this panel that will probably both pop out when you pull this piece off, just make sure you're ready to catch them/find them if they do. you'll probably also have to disconnect a plug that connects to the hazard light control
2.) look for serial number on your radio and write it down. mine was on the top of the radio, it was exposed as soon as I pulled the trim piece off. I think I've heard that it can be on the back of the radio sometimes, so if you don't see it, be prepared to pull the radio out. that's just a couple screws though, nothing hard.
3.) once you've got the serial number, go ahead and reconnect the hazard light plug and put the panel back on.
4.) take this serial number to the dealer, and tell them you need the radio code. they shouldn't give you any crap about this - my nearest dealer is known for being kinda sleazy and they even looked it up for me for free, BUT do be prepared with your license, registration, perhaps even title, that sort of thing to prove you do in fact own the car. they may ask.
5.) assuming you already reset the radio by leaving the key in "on," etc. enter the code they give you. do be aware that for some reason they may have two different codes for your radio (they did for mine) so I would enter the code the 1st time in the lot to make sure it works
6.) you should be all set! make sure you write the code down and keep it where you won't lose it so if this happens again you don't need to go through the whole thing again.