i cannot find the drain plug
Make sure you have a big drain pan...
take out the front bolts and then the side bolts on your pan.
loosen the rear bolts about 4 turns and let the oil drain for a while.
finish removing the rear bolts and carefully lower the pan (you don't want to spill this all over the place).
pour your old fluid into the drain pan and clean your gasket area to ensure a good fit when putting the pan back up.
While this pan is off, change your trans filter. If that doesn't cure your slipping/hard shifting problems, get the trans serviced.
I too agree that those additives don't work.