Go into the sewer via thumb pier (I think that's what it's called...), down the stairs, across the first bridge going up, all the way left, across the bridge going down, then immediately go left, up the stairs. If you haven't beaten the Team Plasma guys in there yet, there will be a guy blocking them, but otherwise you should be able to go up.
When you were in the Sewers during the plot, you may have run into a guy telling you that he lost something and you couldn't go past him (or some nonsense). After you finish up the plot stuff in the Sewers, leave and come back to where this guy was. He was blocking a flight of stairs, and it leads up into the Park.
"I'm gonna match my in game snacking to my real life snacking. Acid. Morphine. Coke. Beer. Vodka." FrooFather on the Fallout New Vegas Diet Plan