If you have all the maps downloaded, copy or cut the .map file to your Halo CE maps folder. I will tell you how to get to it if you don't know.
1. go to my computer
2. go to local disc
3. go to microsoft games
4. go to Halo Custom Edition
5. Go to Maps
7. and paste the .maps file in there.
8. Now see if it works, if it doesn't, then I have nothing else to help you with.
NOTE: You can go to
www.bfmracing.com/maps for all the Halo CE racing maps to download instead of searching through halomaps.org or other sites.