It sounds to me like you are having difficulty figuring out the interface and how things work in the world of Baldur's Gate. I would suggest you take a few minutes to orientate yourself with the game by practicing in the tutorial mode. And to answer your question, I think once you have the key all you need to do is click on Jahiera's cell door and it will open. Then you have the choice of either letting her join your party or telling that snooty druid off (although she is a valuable asset at least for the first chapter for the sake of her healing spells).
--Darkman
PS. Make sure whoever is trying to open the cell door has the key. If Imoen has it, then either give it to your avatar or let her open it
