It's like a bank account. You buy a module and with a higher amount of pesos. Your responsibility to keep track of the time and date. Let's say you wanted a 15 day module of 1mb at 100 pesos but you purchased 200 pesos of airtime. Once the 15 days is up, they will start deducting the money you have left by kb usage which is very expensive. So, if you have finished the 15 day module and you have 50 pesos left, you go to the Super 7 or OXXO and ask for 150 pesos, or 200 pesos and that will start a new module. You have to ask for the module with the text message. They will also send you messages for promotions which really do save you money. Best thing to do is go to a Telcel distribution center and find someone who speaks English and ask them to show you how to do it. We love our Movistar stick and it works just fine. The system keeps track of your usage and you can check it as you are working on line.
All I can add to this is, thank you Carlos Slim. (and I really mean that)