I was looking at maps on YouTube and I always try to download them to my Xbox, but it seems as if there is no longer a file browser for Reach on Waypoint.
Go to your Profile, then Halo Reach Service Record, then click “more stats” and you should be sent to another page and the file share should be on the last tab. Click on it and on the set of tabs below it, there should be a box saying “view public file browser”. Click on that and it will take you to the file browser. (Its a long process I know but its the only way I could get to it).