Air Conditioner

I have a 1998 Ford F-150 with a 4.6l engine. It has 170,000 miles on it. Recently, my air compressor went bad and locked up. A friend got me a used compressor from a junk yard and installed it for me. It was the first time he'd changed a compressor in this particular model of vehicle. I helped him, but know very little about mechanics. After installing the other compressor, we added some new compressor oil and then two cans of refrigerant. The compressor will not turn on (however the clutch and pulley seemed to be fine when turned by hand before installation). I know it's probably not a fuse problem because the old compressor would try to run, but the clutch (I believe) was broken so the belt would just squeal as it tried to run the compressor (I don't have my owners manual so I don't have a diagram of which fuses run each component). Does the compressor need a certain amount of refrigerant to begin running, and if so, how much (the pressure gauge my friend used seemed to indicated everything was fine)? When I plugged the electric power plug to the compressor, it snapped in place and wouldn't come back out on its own, so I think that part is ok. Do you have any suggestions or hints about what it would take to get this compressor running? If the clutch is ok and there aren't any oil leaks anywhere on it, is it still possible the "new" compressor could also be bad? Just a note: when we removed the old compressor, the system still had tremendous pressure which, of course, blew out when we removed the two lines or pipes from it. There was still that much pressure in the system even though the compressor had been bad for several weeks. I don't know it that helps any, but wanted to note it just in case.