Yet again, 343 failed to deliver by giving us a web-based file browser which may or may not come in the future so there are no links to download custom content. Instead you must follow the person you want to download the content from by adding their gamertag as a friend. After that, open Halo 5 and while at the main menu, press X to view your ingame friend list. Click on the name of the player you just added and their file share will be there.
@Jesus in Malibu since 343 did not give us an external download what do you put as your external download link when trying to add a map to the site?