As soon as you enter, make a left. Maybe 3 min walking normal. Entrance will be on your right. Big cave looking thing. You will actually see the track as soon as you walk in.
Manta isn't on the "interactive map" at their website but the PDF version of the newest map, including Manta, is linked from their "General Park Information" page:
seaworld DOT com SLASH assetrepo/documents/Orlando/SWMap%205-7-09.pdf