Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Wondering how to get into the Nightside location? Here's a step by step guide for more Wizard101 fun:
Nightside unlocks after you've defeated Lord Nightshade. Once you've done that, Merle Ambrose calls you back. From there, you have to have beaten the main bosses in Cyclops Lane, Firecat Alley, and Triton Ave. Then you also need to beat Foulgaze in Olde Town. Defeating these four will allow you to challenge Lord Nightshade in the Haunted Cave.
At that point, you need to speak with Merle to receive a key that unlocks the door beneath the Wizard City Commons. You'll find the door under the waterfall near Rainbow Bridge. This is the entrance to Nightside.