First you need to pair your computer with your phone:
1. On Mac go to System Preferences => Bluetooth, click "+" to add a new Bluetooth device.
2. Make sure your phone is discoverable and select it from the list. Use a secure code to pair with your phone. On the phone side and select "OK" to accept the pairing request. If you do not see a popup window on the phone side, simply pull down the notification list on Android. The Bluetooth pairing request should be in the list.
3. Once pairing is done, Mac OS X should prompt you to set up a mobile/dialup connection.
You can select "NULL Modem" from phone Vendor and enter "123" as the number. This will create a Bluetooth DUN network interface.
4. Before you can connect to the Internet, on Mac launch System Preferences->Network, make sure "Bluetooth DUN" is in the network interface list. If you do not see it, click "+" and add it. Again use "123" as the Telephone Number.
5. Click the "Connect" button and your Mac will be online instantly.
That's it! You only need to setup once. For convenience you can also connect directly using the network icon on the menu bar.