Thats is because you dont have a BIS or BES plan. The BB browser requires service books which are only available with those said plans.
If you want to browse the internet with your setup download Opera Mini.
ps: Here's what you are going to miss out on by not having a BIS or BES plan :
MSN Messenger app (you can download third party ones that do work with your plan mind you)
any app that requires BIS connectivity basically.