ac compressor clutch not engaging

i just replaced the compressor, and it is not engaging? does it need to have free-on in the system for the clutch to engage? my dad is also having this problem in his 99 dodge ram 2500

Yes,if is not charged it will not run,or if it is low,there is a low pressure cutout. it is setup like that to protect the comp.

As stated, system must have some charge or low pressure switch cuts out compressor. The plug for that switch can be jumped to make compressor run. Don't leave it that way or the compressor could burn out. Don't forget oil for new compressor.