typically, just like in a battery: hook wires up to the two sides of the membranes (via electrodes) and connect the ends of the wires to a load, like a light bulb. of course, if you try to pull too much current off the cell, its voltage will drop. which means you need arrays of these cells. and the electrode interface is significant, as charge exchange must happen with little loss of energy.
the thing about the proton pump is a little different from standard cells where you are used to liberating electrons. here protons are liberated whilst negative ions drift to the negative electrode. but if you choose that electrode to optimize charge exchange, you will get the electron free of the negative ion, and it will happily flow thru the wire to combine with the lonely proton.
not sure what happens to the hydrogen, need to see specific application.
Les