On forums about nightside, people said that when you're lvl 13 Ambrose gives you a quest to go to Nightside. I'm lvl 15 and I was NEVER in there. When do you go?
Once you finish the four initial streets you will be given access to the last street, and also be given a quest to explore a tower in Olde Towne. Once you defeat the boss in this tower, you will be sent to the last boss...you defeat him, get a key, and then directed to find the area you mention.