on/off usually means that the signal is present at the output connector or not. This is a safety feature to keep you from damaging the circuit you are driving by letting you adjust the generator before applying the stimulus.
The best way to set the levels is by monitoring with an oscilloscope. Some generators have their amplitude settings based on a 50 ohm load. If you are driving the clock line in a digital system this may not be the true load. The safest thing is to watch with the scope and no other connection on the output. Then connect to your circuit and double check again with the scope and make corrections on the settings.