My ac unit is leaking water into my supply box. I had the coil cleaned. Guy said this is all I needed. It's still doing it and running out on my floor. I was told there was nothing I could do. Any idea's why it's doing this? It a downflow unit. It only does it when ac is on.

Condensation drain is probably stopped up take a shop vac to end that's going into where-ever the water is routed and with rag wrapped tight around the pcv suck it dry,,
Should pull out the whatever has it plugged up

I just had this same problem, just remove the drain pipe that carries the condensation away and make sure it is not clogged up. Also check behind where the pipe was installed to make sure there is no blockage there. I was also told that there are these caplets you can buy to stick in the drain pan to stop the build up from occurring, but as I didn't purchase them yet I'm not exactly sure what they are called. Hope this helps.

You could also have a cracked or broken drain pan...

I checked those and the drain is not plugged up. The water is coming off the indoor coil and dripping through the bottom (Supply, I think it's called) box. It is a downflow system. Had the coils cleaned, but still have leak. It looks like it's coming off the fins.

Sounds like a cracked drain pan, sometimes replacements are available but if not you might get a custom built pan made by a sheetmetal shop, remove/get someone to remove and take old one with you