I am metering the hours of a 120 volt pump. does the positive or hot side go through the meter.

Wire the meter with a neutral (white wire) and from the hot wire that runs the pump (typically a black wire). This way, whenever the pump is running so is the meter. Wire the ground to whatever you house the meter inside. I used a metal electrical box.

Can i connect the meter to the ignition switch of a lawn tractor?

You need an hour meter that runs on 12 volts DC. This meter requires higher voltages. This is not suitable for your application. Look at Mechanical Hour Meter Gauge - AIMILAR Professional Engine Hourmeter DC 6-80V for Boat Auto ATV UTV Snowmobile Lawn Tractors.

I have a 277/480 aircompressor will this work?

Yes it should. The pressure switch on most compressors uses only one leg of the either three phase or two phase power to trip the relay to start the compressor, that circuit should be under the max voltage of 250Vac. CAREFULY Put a multimeter on the two wires that energize the starter relay, assure the voltage is below 250Volts, if so that will be the circuit that powers your hour meter. Power when the compressor is on and none when the compressor is off.