To get the messages from Telcel - password or whatever - you go to that same Message box in the banda ancha window and Click to open and then click the message. In the Message box you will have Inbox, Outbox, Draft, New Message. Inbox is where you get messages from Telcel and New Message is where you send them messages.
You have to apply pesos to your account based on the plan you want to use - or module as Chris calls it. You may not always be able to hit it just right. We put in two $200 pesos to get to $400p and use one pesos to send the message to get the $399 plan and our 400 is gone. There are several plans with several amounts so you kind of have to figure which is going to work based on the pesos amounts you can buy at OXXO. I forget the details.
We loosely base our usage on what we use in the states which is usually between 5 - 10 GB a month. We use WiFi a lot and there are two of us on the computers so maybe we use more than some and less than others. But if you buy too much and have time left at the end of the month you lose it and whatever it cost you. Sometimes they have 60 day plans but I haven't seen one this year. Personally I think these prepaid plans tend to cost more but we don't have much choice here. We could by a full year plan but then we would have to pay monthly whether we are here using it or not. One year we got 5GB for 60 days for $400p - and we used it all because we were in places without WiFi. Now more places have WiFi so we don't use Telcel as much. But we sometimes want to get online while on the road and can't get WiFi - even going down the road - or somewhere that does not have WiFi. We do not always get our monies worth but it is better than not having anything - for us anyway.