how do I increase the pressure on my Alpha Boiler?

I have an Alpha he cb25 boiler. The pressure has dropped just under the green mark and I do not have hot water. My gas fitter showed me a lever underneath the boiler and said it takes a few seconds to raise the pressure. I cannot locate this lever. I am confident to do it myself, but I just cannot see which lever he meant? Could some one please tell me where it is and what it looks like please?