Buy the key from Chrysler. They're cut at the national key and lock center for Mercedes. It's in Ft Worth, Texas. It only takes a day or two. They ship overnight to our MB dealerships. My spare key came in a package that had their name and address all over it.

To program the key do the following:

insert the key into the ignition
turn the key to the on position
press the unlock button
turn the key to the off position
press the lock button

You're done! That's all it took for mine.