I stumbled upon this old discussion and dredged it up because I just did the same thing. I put a BlackBerry 9370 on a $30 prepaid plan and only the phone and text messaging works. If you want email and BBM you need the $45 plan at least. That's because these older models (pre-BB10) require a BIS data plan even to access free data over WiFi. You cannot connect to the BlackBerry infrastructure without that plan, and the $30 plan does not include the needed data.
Once BlackBerry released new models (starting with the Z10 and Q10) and the BB10 operating system, the data no longer came through the BlackBerry infrastructure in Waterloo, Ontario Canada, as it did on all past devices. But older models like our Curves still get their data the old way. In order to get the necessary service books for data services to be permissioned, you absolutely MUST have a BIS/BES data plan.
BIS - BlackBerry Internet Service - for consumers/non-corporate users (like you and me).
BES - BlackBerry Enterprise Service - for businesses, and requires a license from BlackBerry plus a corporate Enterprise Server connection.