How do you play user created content on CE?
 

I downloaded the Halo custom edition but wen I tried 2 go on a custom map, it wouldnt work. It just says unable to join game server . do i need sumthin else b4 i play custom maps?

Do you mean online? All you have to do is download the map, put it in the maps directory and run Halo. You then open a LAN server and play a map to test it, and if you're online you go to options and show user created maps. Explain what you did to show this message. Go on a LAN server? Go on the Internet?

When it does that, you have to download the mod first before you can join others with that same mod. Didn't you ask this already?