Q:
I am trying to record my mixes on my computer using USB cable input on top of the mixer direct to one of my USB ports on my pc.
When plugging in the USB cable my pc picks this up fine. I have set the sound set-up as shown in your instructions book with USB Codec being the sound recording source.
I select USB Codec as the recording input. Yet when I play a track the software doesn't pick up audio to record.

A:
Have you connected your REC OUT to USB IN with a phono cable? This has caught a few people out.
The requirement to do this is in the User Guide (page 22), the last sentence of the paragraph below:

USB for Recording
Connect your Xone:42 to your computer using a standard Type A to B USB lead.
Open your chosen music recording software, ensuring that ‘USB Audio CODEC’ is selected as the sound recording device.
You can then send your mix to your computer by connecting Record Out to USB In.

It's possible to read that several times without understanding the implication.
The USB socket isn't internally configured to carry the Record Output signal, but as default is set as a 'patchable send'.
(This is reinforced on page 20 of the User Guide, para 6)

USB Audio I/O
RCA phono. Allows external connection to USB interface.
The routing is configured for external input allowing the USB interface to be connected to the insert point and used as an effects unit with suitable software (such as Ableton Live).
To record a mix using the USB In, connect to the Record Out socket.