The FaderPort 8 needs to be setup as a Mackie MCU Control device as stated in the Owner's Manual:
"Once you have connected your FaderPort 8 to your computer, launch Logic to set up the FaderPort 8 as a Mackie Control Universal device:
1. Go to Logic Pro | Preferences | Control Surfaces | Setup...
2. In the Setup window, go to New | Install...
3. Select “Mackie Control” from the list of control surfaces and click “Add”. Close the device list window.
4. In the configuration menu to the left, set the Out Port to “FaderPort 8” and the Input to “FaderPort 8.”
5. Close the Control Surface setup window. Your FaderPort 8 is now ready to use. Enjoy!"