After looking over my copy again to see if there is any other ways to encourage the players to go down I came up with a couple of ideas:
1. Have the door leading down open and at the start of combat have the players roll a perception check to possibly hear Melaren and Shechera running down the stairs.
2. After combat (or during if they are near the door) let them make a Survival or Perception check to notice a fresh trail of blood leading down the stairs
3. Point out the thing was held down with a Tanglefoot bag and none of the bodies you've searched so far have had any on them so there it's highly likely that there are other people down here that might have some on them (and yes Meleran does have some under his combat gear)
I do like Silbeg's idea of mentioning the pull coming from deeper inside, however I wouldn't give his whole story right away unless the PC's make him friendly. It's nice to leave them with some clues and encourage them to keep talking, perhaps while healing him or something.
Another good way to force the PC's to go down is to remember that there is a vacuum effect when the doors are opened, and have familiars make strength checks not get sucked off their master's shoulder (which is where they usually sit). Granted when I ran this having the Witch's spellbook roll a -2 on his strength check caused him to almost die from Skeletons...but it did a great job forcing the party to rush after him!
So you could have Meleran while muttering about the pull walk over and just lightly brush the button next to the door causing it to open since they are swift actions in here I'd say a player couldn't stop him unless they are actively guarding the panel. Since it only causes Severe winds on small PCs and familiars will be affected.
Also since that door is the first one to have a vacuum effect the party pretty much won't be expecting it.
