okay, first of all...you're not dumb.
Lots of people have trouble at this point.
Have you completed the crypt? Have you found Scabban? If you found Scabban, then you'll notice that there's a hole in the floor of his room. Flip the lever on the wall behind him and then dive through the hole. You'll fall through to a underwater chamber with a tunnel leading south (?) I think. Swim through the tunnel (fast, because most of your characters at this point can't hold their breath too long) and emerge out in another watery area. You'll be in a room that opens onto a set of canals arranged in a diamond shape. That's when you're in the Tomb of Heros.
-Sazerac