if you are asking how to just get to the bombsight then, as Murdr already said, press F6 when you're in the cockpit.
if you're asking how to use it then when in the cockpit press F6 to get to the bombsight. press U to get to calibration mode, hold in Y for 2 seconds or more to get better accuracy, then press U again. However the calibration will only point to the spot on the ground for the speed and altitude in which you calibrated at. So if you press U, hold Y, and press U again and you are going 220 mph at 10,000ft then that is what your bombsight will be set for. If your speed increases/decreases after that to lets say 180mph and your altitude drop/raises to 8,000ft then your bombsight will still be set to 220mph at 10,000ft and therefore, if you drop without calibrating again, you will miss your target. The best way to avoid this is to pull your throttle back a little and level out a good way from your target so your bomber will be at a certain speed and altitude will stay basicly the same the whole way in to target.
