Ok, let do this.
So, you said you downloaded the maps. Correct?
Well, make sure you put them in your Halo CE maps Folder.
If you've done that then your fine.
Although, the Map Manager doesn't work for me, so I download the maps individually. I suggest maybe doing that to see what happens.
When it says unable to join game server, then 99.99% of the time, your missing a map.
Also, you do not need a password to get into the server.
I hope this helps some.
~Brick