"Sulu? Last I checked, he was going with the Enterprise to check out a wormhole... Apperently the probe sent through didn't give much, other than saying something about a galaxy far in the past and a good distance away. I dunno. All I'm really worried about are the tribbles I was bringing as a sarcastic gift to the Klingons here."
Well its no wonder that you cannot find him, because he is actually dead. The Sulu being mentioned, by several NPCs, to stand in Admiral Quinns Office and the Quest text is just an Undine infiltrator. Guess you have to drop the quest now.
Alone in a World that has already ended - building Cyborgs out of trash.