There's tool that come with the game, these are installed only if you select to have them installed when you're initially installing Halo 2 Vista there's an option for "map editor".
If you have a Halo 2 Vista disc, just put in back in your computer and the install menu will appear, just select install map editor (or something along those lines).
Or you might be able to download the Unlocked Edition of Halo 2 Editing Kit, which should be somewhere on the repository (as linked above).
The unlocked version allows for more control/editing ability with the tools.