do you know how to play coop? if not you can find a video about it
here
if you want to play on LAN, cartographer supports this but you'll need to edit ini files to be able to do so.
Type this in your start menu
%localappdata%\Microsoft\Halo 2
Find halo2config1.ini and open it with wordpad or notepad++
Scroll down past the comment lines and find the "WAN_IP" and "LAN_IP" lines. These need to be filled in on each pc on your network. WAN_IP is your IP address found on
http://www.whatsmyip.com - LAN_IP is your local IP (on your network) which can be found by typing "ipconfig" in the command prompt.
Once these settings are filled in, make sure every pc has a different "base_port" setting, with the difference being at least 10, so by default the mod uses 2000, 3000, 4000, 5000 etc.
hope this was clear