First, the pump has to be below the Mash Tun outlet. And you have to arrange the ports so that the pump will fill up completely, otherwise you will cavitate the pump.
Lastly, you have to have a valve on the OUT side of the pump so you can choke the output. Otherwise the pump will draw too hard and starve itself due to the grain bed restricting the flow. Just open the valve a little to get it flowing. Once you have a circuit going, you can adjust the flow to your liking, or to the rate at which the grain will allow.