I don't think Harry told him anything. Harry didn't care about MJ and Peter at that point. The big glowing warehouse out on the pier probably gave away his position.
Because Harry didn't yet have the tritium. Do you think Harry keeps 5 pounds of tritium in his wall safe at all times.

Harry said go get Spider-Man and I will get you your tritium. Plus, Ock wants to get rid of Spider-Man so he doesn't interfere with his work. It's a win/win for Ock. Get rid of Spider-Man and get really expensive and really rare tritium.
The only thing Doc Ock knows is that Peter Parker and Spider-Man know each other. Doc Ock doesn't care if he kills or hurts Peter. If he kills Peter or hurts him really badly, then Spider-Man is going to find out and come after Ock. Ock then takes MJ as hostage to use as leverage. He isn't delivering a message to Peter Parker, he is delivering a message to Spider-Man.
Plus, Ock is just creating mayhem. He didn't know they were in the cafe. The car happened to land in the cafe they were at. You can tell that because Doc Ock is surprised to see them together in the cafe. Listen to his tone when he says: "Peter Parker...and the girlfriend!" If he knew that they were in the cafe together, he wouldn't have sounded surprised to see them both. He was just trying to make Spider-Man or Peter, who is a photographer, come out in the open. Peter Parker takes pictures for a living, if Doc Ock is out creating mayhem, Parker is going to come out to take pictures or Spider-Man is going to come out to fight him.