Instructions
Open the map you wish to edit in "Forge" mode. You can open Forge from the "Halo 3" main menu.
Press up on the directional pad to enter Editor Mode.
Hit the Start button to open the main Forge menu.
Select "Change Game Type." Scroll through the various game types until you find the one you would like to work on.
Wait for the game to load a new round. Once the new round begins, your creation palette is stocked with the correct goal items for the game type you selected.