I have yet to buy the pass card because I do not understand how to use it is written to go in the shop Halo 4 during the game but I do not know where I can find the store
On the Halo 4 Main Menu, there is an Xbox LIVE Marketplace option.
When you have the Halo 4 War Games Map Pass installed, choosing this option will prompt you to download the Crimson, Majestic and Castle Map Packs.