How do I get to the small ledge imbetween the two ladders after I take the part?

I assume you've just recovered Azerim's part and have found treasure #3, a golden idol. You have climbed back up the ladder and are facing a ledge in the doorway facing the shaft between the two ladders.

Try this: line up with the doorway and do a forward roll(Z-UP arrow) onto the doorsill. It worked for me. Maybe you should save your game here in case you don't make it the first time.