Hey Will... I had a very similar overflow for a while. This one is similar to the CPR style, which uses an aqualifter pump to start/maintain the siphon. There's a small nipple at top of the black section where the pump is connected. You fill the tank side of the overflow box with water, start the pump, and it pulls the water up the tank side of the overflow and starts/maintains the siphon. The one that I had did not come with the pump attachment so to get mine started, you would insert a section of airline tubing into the unit from the tank side, up and over the bend at the top, fill the tank side of the overflow box with water, and suck on the other end of the airline tube until you pulled water up and over the top portion of the box. A little hard to explain, but it works.
Hope it helped.
Now... get some coffee.
Mitch