To have fader control over the MixBus outputs on the X32:

1.  Navigate to the ANALOG OUT tab of the ROUTING screen.
2.  Use the first encoder under the display to scroll to the output you wish to have fader control over.
3.  Verify that the output is selected to the appropriate MixBus.
4.  Set the TAP of the output to be POST FADER.

This tells the output of the X32 that the signal is being pulled to the output after the MixBus fader, which will provide fader control over that signal.