Look here.
https://forums.halowaypoint.com/yaf_postst129742_Halo-4-Support-FAQ.aspx
They state…
I can access my File Share in-game, but I can’t find it on the Halo Waypoint website.
Web access to your Halo 4 File Share will come online next month. Your in-game File Share, which features 25 slots, is fully functional in the interim.
everybody calm your farms about this, and wait till next month when all the awesome custom content will be easily accessible.