If you have the AR code for celebi how do you get it in the game?

If you mean how to get the code in your game you need an Action Replay which you can buy for about 20 dollars. First you have to update it before you can use codes on Pokemon Black and White. You can find out how to update your Action Replay by going here. Once you updated it you can add a new game on your Action Replay. Use the code the Action Replay gives you when you turn it on with your Pokemon Black/White game in it. The M code is also the game ID which you get when you turn on your Action Replay with a game you haven't added yet. It shows up where it says Unknown Game. You can also make a topic in that forum if you have any more questions.

If the code is for another game and you plan to transfer the Celebi over to your Pokemon Black and White game, then you need to use the Relocater. You can unlock this option by going to Castelia City. In one of the buildings is a guy who will ask you for a phrase. Answer with "Everyone Happy" "Simple Connection" this unlocks the Relocater on the game's menu. To use it though you'll have to have two DS systems.