Microsoft and Bungie have solved whatever problem was preventing people downloading Halo 2 DLC ahead of the loss of Xbox 1 support on Live in April.
In order to access the various map packs in time for the FPS game's grand send-off in two months' time, just fire up Halo 2, make sure you're on Xbox Live, and find the Content Download option through the menus.
This news item clearly isn't long enough, so here's a gratuitous link to our Halo 2 gamepage, where you can read the original review and find lots of other relics of Xbox Live's first huge success story.