OK, I got past it.
Here's the deal:
Go to BTMRR
Go into the middle door at the bottom
This will lead up to the next level.
Walk out
Walk back in -- you will enter Injun Joe's cave.
From there it is a maze to get out on top.
From memory (since i can't get into VMK right now) this part gets a little hazy:
entering the cave, go down the waterfall.
Then come out at the top, go around to the right and doulbe back.
Then come out at the bottom and go out to the left.
Then walk across BTMRR at the top into the house.
From there you go back through the cave one last time and out to the right onto the top of Tom Sawyer island.
I hope that's right. I did it enough in the game this afternoon that if I could see it I could be sure.
BTW, The "Mark Twain" thing was not an answer to our questions about how to get up there. It was people answering a later quest question. No one could be bothered to explain this, of course.
I ran into many in the Mark Twain room thinking they told to go there to get up top. . .