When you start Halo CE and it is loaded and ready for play select throught these screens.
MULTIPLAYER / JOIN GAME / INTERNET
You will then come to the Internet Lobby that has the powered by gamespy logo in the top right corner. At the bottom of this screen you will see a button labeled FILTER. Select this button and you come to the Filter screen options. The top 3 options can be whatever you wish. However the bottom 3 for racing on the BFM HALO CE server should be set to the following;
GAMETYPE = RACE
TEAMPLAY OPTIONS = TEAMPLAY
SHOW USER CREATED MAPS = YES
At this point you should be fine and ready to race assuming 1 important factor, You downloaded and unzipped the map files to the following directory:
C:\Program Files\Microsoft Games\Halo Custom Edition\maps
(I personally choose to download the zipped file to the same directory as the rest of the map files to easily locate them)
After unzipping the files to the above listed directory, and restarting HALO CE, you should be good to go. Hope to see you on the BFM RACE/CE Servers.