I really recommend (again) that you visit the ICE website. Here is the link:
http://www.grupoice.com/wps/portal/kInternet_prepagoYou will find the answers to your questions there.
It is possible, and somewhat likely, that someone at the ICE boothe at SJO can help you.
To buy unlimited access you send a text message to 6060 with the word for the period you want to buy. (ie dia, semana, quince, mes) You must already have sufficient funds in your account to pay for the service when you send the text. In other words, recharge before you buy.
It costs ¢9.000 per month. I usually buy 2 weeks at a time. The cost is the same (¢4.500 X 2) per month.