We have charged the new air compressor, but the compressor will not turn on when I turn on the A/C switch (the clutch will not engage). Can anyone help? My husband and his mechanic friend have checked everything.

Evacuate the A/C refrigerant and measure how much is removed.
Accurate system charge is vital to proper operation.

Like Rich said, if the freon is low, the compressor will not turn on. Also, did you put the right type of freon in the vehicle? Your vehicle requires R134a freon, not the R12 which was used in older cars.