Does anyone have any experience adding coolant to the ML 320? If so, do you have any suggestions or advice for someone attempting it for the first time?

Thanks!

 

You mean refrigerant?
Is there little bit still in your system? Compressor comming on but air is not cold enough?
For the first time I would try it on some other car instead of your Mercedes.
If the system still have some pressure (not completely empty) you can add more refrigerant using appropriate valve and pressure gauge that would show the state (level)of your system charge.
If your compressor is not turning on, your system is empty.
you need to find the area of leak, repair it first, connect vacuum pump to get out air and moisture from your system and then charge it with propper weight of refrigerant.