Hopefully this helps clarify,
"Who is eligible for this free content?
"All online users that have played “Halo: The Master Chief Collection” between Nov. 11, 2014 and Dec. 19, 20141 will be eligible for the 1-month of Xbox Live Gold, an exclusive in-game nameplate and avatar, as well as the campaign for “Halo 3: ODST.”
Content and 1 month of Xbox Live Gold will be provided to all players of the “Halo: The Master Chief Collection” game on any day from launch on November 11, 2014 through December 19, 2014 at 11:59 pm PST (all Gamertags that have logged activity in the game via Xbox Live during this period will be eligible). Xbox Live Gold will be delivered as early as mid-January 2015. Offer is non-transferable"
I believe as long as you were connected to the Internet while playing MCC whether multiplayer or single, you get all the perks.
I fully installed and did my settings for the game yesterday, so doe that count?