Trans fluid is extremely detergent, and pretty much cleans out the cooler itself. But, if you want all of the old fluid out of th rad., take a piece of an old line and tighten it into one of the cooler lin holes, remove the other, and hose clamp a piece of 3/8" fuel hose over the piece of steel line. BLow through this with your air compressor on low pressure slowly increasing until you get only air. Trust me it makes a mess, but it works.
THe other way would be to go to a repair shop and have a trans tech run on the cooler. It hooks up to the cooler lines and forces old fluid out and new fluid in...this thing has virtually eliminated the tranny service of old. It changes 100% of the fluid. Take my advice don't let those a**holes sell you any additives. You dont need them. the fluid is detergent enough. Additives=slippage