Setup bluetooth between phone and macbook and make sure they are connected. then under network preferences you should see a Bluetooth (Modem) on the left. click on it and enter *99# as the Telephone Number. Leave Acc. Name and Password empty. Then click Connect. Voila.
Sorry, no free internet. As soon as you connect your blackberry to the pc/macbook the BIS icon disappears and it says Modem Mode Enabled. So you get charged for any data usage.
I can connect via both bluetooth/usb with my bold (os 5.0) and macbook running snow leopard. Didn't install any software.