Are you romancing her? That makes a difference in how the quests play out. SPOILERS
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;They begin the first time you try to travel outside of Athkatla. You've receive a mini-quest to return a poisoned Harper back to the Harper Hideout in the docks district. Then you'll get a quest from Xzar, who is standing at the base of steps leading down to the bottom area of the docks. If you complete Xzar's quest, then you start to get a string of events about your party's interaction with the Harpers.
If you've been playing the game for some time after completing Xzar's quest, and you still haven't had any of the other Harpers quests, you may have one of the manymanymanymanymany bugs associated with Jaheira and the Harpers, which seem to be a much bigger problem if you're actually romancing her, but can exist even if you are not.
There is some discussion of this problem on several different threads. If you're having the same problem I was, you may find the answers you're looking for at
http://www.gamebanshee.com/ubb/Forum2/HTML/002345.html
Hope this helps