Proper way as explained in the thread:
Once you buy it go to purchased items and download the dlc pack. Once downloaded chuck the zip itself (do not try and unzip) in one of these folders depending on OS:
Win XP: My Documents>Crysis 2>dlc
or
Win 7: Username>Saved Games>Crysis 2>dlc
Then start the game up and log in as normal and the maps should be available in your server browser.