Hi there you can't open that building I think it might be an addon pack option for later. The items for the garden are spread all over Dorns deep and Wyrm's tooth. It will riun the fun if I tell you exactly where they are but its worth the traveling time. Once you find the seeds, and animals if you save the game just before entering the garden. Then give the guy the stuff and collect the exp, export the characters. then reenter the game with the new exports and do it again you'll gain some levels without expending spell scrolls or arrows.
I have a humble little website if you'd like to visit it
http://www.iwdbg.cjb.net/
If you ever want to try an online game I'll be happy to show you around the Savage North