Please note that this tutorial was written for Logic Pro 9 and although the basic principles are likely to remain the same for future updates, some of the screenshots may not exactly match up with what you see in future versions.
To set up an Audio Interface in Logic:
1. Ensure that you have correctly installed all relevant drivers for the audio interface (if applicable). Check the product's website or manual for full information.
2. Make sure that the interface is connected to your computer. This is usually achieved via a Firewire or USB cable.
3. Navigate to Logic Pro → Preferences → Audio in the Logic menu bar.
4. Select the ‘Devices’ tab, followed by the ‘Core Audio’ sub-tab.
5. Use the ‘Output Device:’ and ‘Input Device:’ drop-down menus to select your preferred interface.
6. Click ‘Apply Changes’.
Your interface should now be set up for use with Logic Pro, i.e. you should now be able to record into Logic using your interface and all of Logic's sound output should also pass through the interface (e.g. out of any speakers connected to the outputs of the interface).