You must have a blackberry data plan to utilize the blackberry/RIM data functions. (It is possible to do some data functions with a generic data plan, but not blackberry data.) This is in addition to the voice plan, but obtained from the same service provider, assuming the service provider supports blackberrys.
See What do I need a Data Plan for? - BlackBerryFAQ
If the blackberry was purchased second hand, see Buying a BlackBerry off eBay - BlackBerryFAQ
. It doesn't apply just to ebay purchases; and if you are having problems, your answer may be explained in the article.