I have know idea how you can possibly get there as free but I know that free people can teleport into you if your in there because my friend hasnt subscribed and she teleported right to me while in Nightside..... so I dont know. It doesnt make much sense because to do Marla Stewart's quest you have to go into Cyclops Lane. Why would they make it free?????
But there's no way for you to gain continual access (via the quest key) without purchasing some content to the game.
Nightside is at the "end" of Wizard city, and because you'll need to either subscribe or utilize access passes to complete the area, technically it's not "free".
All I can suggest to you, is to either subscribe or purchase access passes. It's a great game, and the developers deserve income! Not all things in life are free. And some, are worth paying for; I feel this is one of those moments.