Looks like you may have to contact the phone company and get a plan to do so with the blackberry. Without paying them for the privilege, you may be violating your usage agreement if you tether it to a computer.
Tethering doesn't always work as a hotspot, by the way. Some phones can only hook up to a single computer via bluetooth, and have to have special software on the computer to make it work. I'll keep looking into it.