If you got a new map, its not a mod its a new map made for halo ce (custom edition) so you need to get that to play it, search on google for halo ce.
If you got a new map mod thing, then if its the whole map, just copy the original map to somewhere safe, and put the mod in the map file directory, when ur fininshed playing pu the original map file back in, like from where u placed it safe.
If the mod is a PPF file the download PPF o Matic from like google search it there, then open it up and get the map you got the mod for, put it in the the first box and select your mod in the second and hit burn, it will create your moded map, rememebr to backup all of your map files.