Setup is easy. On the N900, download and setup the app via Application Manager. Done.
Pair your phone with your Mac which makes available Bluetooth PAN and Bluetooth DUN network adapters. Using the Bluetooth DUN adapter enter the settings shown in the screenshot below. The Password is CINGULAR1.

In Advanced Settings I have the following options under the Modem tab:

Press Okay after making your changes and press Connect. Voila! You're connected and surfing through your N900.