It's the Silent Cartographer (Map room, 4th level) for those of you who don't know what map he's talking about.
You need to go back out (after they close the door) and as you head out go down to the beach and head left.
Cortana will make some comment similar too
Hey look a path to the interior of the island!
There will be a hill to jump if you want to land on some covenant.
If you can't find it that way, after you first clear the beach and Foehammer delivers your warthog, go back to the start and go around the island backwards. When you find an overturned warthog, that's where the entrance is.