Wow... same here. I've been waiting for $400 travel credit to appear (annual fee already paid weeks ago), called yesterday and they adjusted something and promised it would show up.
Checked numerous times after that yesterday and not showing in either the travel website OR the My Redemptions page.
This morning, some progress, the $400 travel credit appeared in My Redemptions under Gateway account, but still NOT in the Travel website.
Finally after reading just now that yours has appeared, I checked again and BINGO! $400 travel credit now showing in the travel website and ready to book 
If anyone is waiting on their $400 explorer travel credit - if you don't have it yet, call up and ask for it to be added manually... then wait some time for the system to catch up. Mine took around 24 hours from phone call to having it ready to use.
