I own crysis 2 on Origin. I just bought it on steam to get the DLC maps. I installed the steam version and it found all the DLC (new gun, dog tags) but NOT the DLC MAPS!
I've done the redeem code which takes you out of game and it says either web page not found or promo code not found!
I'm about to call American Express to cancel.

Just did again. Here's what it says;
"Order Received


Promotion code was not found

Home Purchase History
"

Another user just reported a similar problem -- it might be a new one that is surfacing with this weekend's Steam sale of Maximum Edition. Here is the thread, does this seem to fit your symptoms as well?
http://secure.gamesas.com/forums/viewtopic.php?f=40&t=55418

Aside from entering the CD Key there is no special code to redeem for C2ME... Using the multiplayer menu option gamesas -> Redeem Code is not necessary and will fail.