You can add -devmode in the target line of the shortcut of Halo on your desktop...

Or you can download bitterbanana's devmode programmy and just turn it on...
Link

To enter the singleplayer map, press the tilde button (the ` or ~) and type "map_name <map name here>" without quotes of course and you type in the map's name in that area, usually the readme included with the map tells you what to do.

[Edit]
AHHHHH!!!

Well at least I told him how to play the SP levels =)