I have the exact same diffuser that I use in my 30G tank with pressurized CO2 and it does a great job. Don't worry about filling it with water, the water will get into it when you submerse it. As long as you have sufficient CO2 pressure pumping into it you should see a very fine mist of CO2 bubbles coming out of the top of it once the CO2 starts flowing.
Are you using pressurized CO2 or DIY?