To download Forge maps in the Master Chief Collection, you'll actually need to look the author up in-game. It's a bit annoying but is the only way, unfortunately.
- Launch MCC.
- From the main menu, press [X] to open the Roster panel.
- Select yourself with [A] and navigate to 'Find Player'
- Type in the gamertag of the author of the map you wish to download.
- From the user's profile page, navigate to 'File Share'.
From there, you'll be able to view all of their maps and other files and save them. Hope that helps!
Also, welcome to HaloCustoms!