I believe you're thinking of the Merchant League Headquarters, not the Seven Suns. (Easy to get them mixed up: They're the same general type of building, look roughly the same from the outside, are quite near each other on the exact same map, and have the same type of--um--inhabitants.)
The door to the Seven Suns shouldn't be locked at all, but you CANNOT enter the Merchant League HQ--ever--unless you accept Aldeth's quest.
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Volothamp's Comeuppance
Everything you ever needed to know about the entire Baldur's Gate series......except spoilers.
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