Pressure cooker missing a part. How do I fix it?
It's good that the "green arrow valve" never popped up; that's the safety fuse, and only "pops" (destructively, and probably spraying food onto the ceiling) when the cooker pressure goes way too high.

The missing part is the pressure regulator, gauge or counterweight. The pressure cooker won't work without it. It blocks steam coming out of the vent until the steam pressure gets high enough to push the regulator up off the vent, thus limiting the internal pressure.



These are usually pretty easy to replace, as they are easy to lose.

It's important to get the right one, to use the wrong one could be very dangerous!

Your cooker is made by Presto. Presto calls that part a pressure regulator, as well as I can tell: http://www.pressurecooker-outlet.com/Presto-Pressure-Cooker-Parts.htm