If it’s a one-pipe steam radiator, the vent belongs on the side of the radiator that’s opposite the pipe. Because the lighter-than-air steam will head first for the top of the radiator, you should install the air vent about halfway down the radiator, and not at the top.
Two-pipe steam systems (with the exception of that “missing link” one-pipe, air-vent system) should not have air vents on the radiators. If the two-pipe radiator won’t heat without an air vent, check the steam trap. Misapplied radiator air vents can lead to nightmarish system problems.