Never used the kit myself, but as I understand it, the tablet fizzes releasing the CO2 which is trapped in the 'diffuser' and then slowly dissolves into the water. If it is, you'll need to add another tablet each time the CO2 in the diffuser runs out, which should be obvious.
Sorry to have to say this, but that kit is pretty useless, there are much better kits available, like the nutrafin yeast one, for about the same price. I also doubt very much if that sera kit works for tanks up to 200lt as claimed.
Sam