It is behind the the input/output knobs at the top right of the heater. The reset button is right at the back on those connections, it's bassically faces the back panel for the heater, there should be more than enough space forn you to put your finger behind there. It's a little white button. You will need to give it a good press for it to click.
Hope this helps. I had two of these to reset on thursday, xmas eve!
Troy, December 2009