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Go down to the Wards, find the stairs down to C-Sec, they're before the clinic. It's in the big room where you met Emily Wong.
Go down the stairs, enter C-Sec and in the center of all of C-Sec there will be a big elevator. Go down that elevator and you should meet Udina and Anderson at the Dock.

Or you could go from the Presidium. Right below the embassies there is a small road that has an elevator down to C-Sec, use it and when you get down to C-Sec the elevator to the dock will be in front of you. Follow the signs on the presidium if you can't find C-Sec that easily.

u can get to the Normandy through the police HQ on the citadel. take a lift from the level where u got ur spectre gear. find the police HQ via the map.