That is a loaded question
What point are you in the game? Have you completed spellhold? To get to the underdark you have to start the game in motion beyond chapter two. I really don't want to be to specific b/c I don't know what you have done and don't want to spoil it for you......but let me know where you are in the game and I will help if possible.
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It is better to keep your mouth shut and appear ignorant,
than open it and remove all doubt!