Single player maps and replacement ui.map errors

[wingsls]

Hello
Here's my issue, I'm trying to play Halo CE (Custom Edition) in a single player mode. I have downloaded many single player maps. I have replaced the ui.map to correspond with the single player maps but when I launch Halo CE, I receive this error:

"We have detected that one of the Halo PC files is missing or corrupted. We recommend removing and reinstalling the game. (maps\ui.map)"

I did backup my original ui.map file so I still can play the campaign and multiplayer maps.

But even when I use the console when playing Halo CE with the original ui.map I receive this message when I try to change the map thru the console.
I type in at the command line: halo< map name hugeass

and it replies: requested function cannot be executed now.

So can some one give me a DETAILED instruction as to how I can play in the SINGLE PLAYER mode and what I need to do to get everything to work?

Thanks in advance