The small button for the door in the cellar of the White Witch's cabin is right left to the door itself. Proceed there, that's the right way.
Have fun playing.
You also might have a look at my travel guide. It explaines the "best" path for the game if you want to meet balanced enemies:
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http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=513032#Post512459
