I just checked their directions and it's even easier than what I did below.
To vent the system:
1. Turn on the water supply to the hot water source.
2. Make sure that there are no leaks in any of the connections.
3. Turn on the tap or faucet furthest from the hot water source until there is a steady stream of water with no evidence of air in the system
This was mine.
The easiest way is cut a tee into the line near the last check valve, add a valve that can open and close. The last one I did I used a angle stop for a faucet, that let me thread on a lav supply so that I could direct the air and then water into a bucket until I had bled the air out.
They want the air purged so that the pump isn't running on air.
And back to their instructions