If you do not currently have AIR installed start with the TweetDeck installation post to get the required 32 bit dependencies before following these instructions for adding AIR 2.0. More here

If you followed one of my previous posts for installing TweetDeck or Seesmic in Ubuntu 64 bit you are currently running AIR version 1.5. You already have all of the dependencies needed to run AIR on a 64 bit system so upgrading to AIR 2.0 is quite simple.