In order to enter the sewers, you have to talk to a NPC in Prontera, to be "part of the Prontera Culvert Rescue Crew" or whatever, lol.
Well, go into the big building at the upper left corner of Prontera. In there, you need to talk to a NPC called "Recruiter", I think. He'll automatically send you to the culvert/sewers entrance.
By the way, you'll find the entrance when you leave Prontera to the left, then walk westwards until you see the small river. The entrance should be near the bridge.
