343 and Microsoft have finally released the ODST campaign for owners of the Halo: The Master Chief Collection. If you owned the game (and played it) when it originally came in check your Xbox Live message center as you might have a code to get the campaign for free (as a thank you for all of the cock-ups the game had at launch). For everyone else the game will cost you $4.99.
Here's a short overview of how to find the content as well as a short overview of ODST.