If you played the MCC between launch day and December 19th, check your messages on your console for your ODST code.
If you don't qualify, the ODST campaign is available for $4.99 from the Xbox Marketplace.

Relic along with a decently large patch (2.4GB) are now available for download the next time you log into MCC. Make sure to download the patch before Sunday!

Patch notes: http://aka.ms/may30update

FAQ on Relic on ODST: http://aka.ms/odst_remnant_faq