Wifi and Blackberry app store help

I have a pearl 8120. I have downloaded the App store via the wifi connection. but when i click on the icon to use it is says.

Upgrade your service to a plan that inclueds browsing and discover a world of possibilities at Blackberry App World. contact your service provider.

with wifi i am browsing. i have a full email package so i get data and the phone has wifi. i do not have an internet package.

why should this matter if i can browse via wifi???

can anyone help? I have an unlocked 8120 tmobile phone i use for my Rogers account.