That's because it's a singleplayer map. If you try and start a singleplayer map in multiplayer it will always cause an exception. To play singleplayer maps in CE you have to launch them via the console.(In-fact there is a note about this right on Halomaps) In order to use the console you have to run Halo in devmode.
How to play SP maps:
http://hce.halomaps.org/index.cfm?nid=386 (The second method on this page only works with the original campaign maps)
How to run Halo in devmode:
http://hce.halomaps.org/index.cfm?nid=319 Additionally, if you installed the HEK, it automatically adds a devmode shortcut in your start menu.
Edited by rerout343 on May 24, 2012 at 10:05 AM