Their site tells you that you need to fix the lean condition problems with the following device:
MAP Sensor Enhancer: When the HHO system improves the combustion quality, the oxygen sensor in your car will see much more oxygen and begin injecting more fuel. When this happens you will probably not see any mileage improvements due to the computer fighting against the Water4Gas device. The MAP Sensor Enhancer allows you to adjust the fuel injection and the amount of fuel going into the engine. The enhancer is very important for fine tuning your HHO system.

It appears to me that you are messing with the ECC and bypassing it for altering fuel/air ratio mixture.

Matt in Moore Tafetta White 2008 HCHII