I have a 30amp 240v line running to my central A/C unit. And I want to install an 20 amp 120v outlet next to it to run an air compressor unit. Is this possible and how is done?
No. The National Electrical Code does not allow line to neutral loads on a 240 volt line.
Besides that the line to the air conditioner will not have a neutral for the 120 volt line anyway. If you cheat and use the equipment ground that is another very dangerous code violation.
Just run a new wire. It is not worth you and your family's safety to save a few dollars.
Good luck and stay safe!