That guy did you and his store a disservice...idiot.
If you use DME for priming you'll need 1 1/4 C of DME boiled in 2 C water for 5 mins.
If you prime with Corn Sugar you'll only need 3/4 C.
Add it to your bottling bucket before racking from the secondary. It'll mix during the transfer, but I always give it a SLOW stir or two, bottle about 10 bottles and re-stir.