How do I get compressor to engage to add refrigerant to it?

When you are putting the initial charge in the system it should meet the minimum charge to trip the pressure switch and the compressor should engage. Here is a guide that should help with this:

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

If this does not happen then you can just jump 12 volts to the compressor clutch on the red wire. This will engage the compressor and it will start pulling in Freon. If the compressor does not engage then you probably have an issue with the compressor.

Lastly if you jump the compressor and fill the system to the proper charge and it still do not come on then you may have an issue with the pressure switch.

Let me know if you need more help do this. Thanks