Your map has to be in the map repository for the ingame download to work. Ask someone who runs it to add it there. Preferably your map should be finished before they put it in the repo. Hope this helps.
You can still send the map file to your friend and put the file manually into the custom maps folder, which is This PC/Documents/My Games/Halo 2/Maps. You can only have 50 maps in there tho, otherwise none of the custom maps will load in the game.
