Do you have the Limited Edition of Halo 4 or perhaps you've purchased the game's map pass? If you have either of these you may also have had trouble downloading the new Crimson map pack that released just yesterday.
Today I popped in my copy of Halo 4 to download the pack but was greeted with the request to pay additional Microsoft monies for what should be a free download since I've already redeemed my Limited Edition codes. If you're struggling with a similar issue, there's fortunately a simple solution.
Reboot your 360. And be sure to navigate the in-game Xbox Live Marketplace for your content, not from the 360's dashboard.
Relevant side note: some commenters on this Joystiq article reported that they were able to download the map pack for free regardless of what edition or pass they'd purchased. My guess is that 343's solution to the Limited Edition/Map Pass owners' issue—namely, creating a second download option for free for these users—was the cause for the unanticipated bonus for the standard edition players.
[Update]: Microsoft PR informs me that the free access to the map pack was intentional as a limited time thank you after complications with reward tokens. Here are the details:
As an additional reward to those customers that participated in our "14 Day, Buy and Play" program (which ran from Nov. 6 – Nov. 20), we are providing complimentary access to the Crimson Map Pack for a limited time, through Tuesday, December 18 at 9am UTC/GMT.
There were some initial complications with the distribution of the "14 day, Buy and Play" reward tokens, and so we decided to provide the additional reward of complimentary access to the Crimson Map Pack, for a limited time, as a "thank you" to our customers that contributed to the success of the "Halo 4" launch.