If you have the EVO then it comes with a guage on it. That guage reads the pressure output from the inline regulator (the foregrip one). If you have the stock PMI reg the smaller setscrew on the bottom of the reg adjusts the pressure. Screwing it out raises the pressure and screwing it in lowers it. Note that if you lower the pressure you need to shoot two or three times to bleed off the existing pressure in the valve area so that the guage can read the new setting correctly. But for raising it you don't need to do that.
Setting the velocity is about balancing the output of the reg and the tension on the hammer spring. Generally they won't shoot well at much under 300 and seem to work best at around 350 on that little guage. THen you set the spring using that knob to fine tune the velocity at the chrono when you go to the field..
Did you not get a manual with your EVO? It will explain a lot in there.
And here's a few links to places that can help you understand a lot more about the parts, their names and how it all goes together.
Have fun. It's a great marker.