If your carrier t-mobile? If so, you do need the internet plan I'm almost sure. My brother told me like the whole system and he said that in order to use wifi, we needed the internet plan >>;
I'm with T-mobile and I also have the 8900. I don't have the internet plan but I can still use the wi-fi. As long as you have a wifi network to use, you can access the internet with the wifi. For example, I have a netgear wifi network in my home and I can connect to it through my phone. You just turn on your wifi through "Manage Connections" and you can go to "set up wi-fi network" and then scan for networks. once you are connected with a wifi network, I go through wi-fi hotspot login under the set up wifi network and then it says " Your Blackberry device is now connected to the internet"
boxofcrayons, sorry if that didnt help but its the way it works for me. hope it works out for you though.