OK, I've got the stupid blue marble but how do I use it on the door? I've tried using it and clicking on the door, and using it and clicking on the pedastal in front of the door. But it ain't working. Am I doing something wrong???

there isn't a pedestal in front of the door that you're supposed to use the blue marble on. try looking for another door to use the blue marble on.
spoiler below
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the pedestal is for the idol that you nab from marten's bluff. the blue marble is for you to use to get into the 7th bough. it has a blue circular design on the door.

You're in the fifth bough area, if you're at the pedestal. Go back to the ladder, go down and go across to the fourth bough. There, you'll see a locked door. Use the blue marble on it to open it up. Be ready for some tough fights in there.