Official comment
Your sound card should show up in this dropdown menu:

Comment actions
Hi,
I have a Pioneer DDJ-RB Controller running Rekordbox and would like to know what I need to do in the audio set-up to run it through a Scarlett 2i2 soundcard.
Thanks.
Your sound card should show up in this dropdown menu:

Post is closed for comments.
Thanks for the reply.
Once I select it what needs choosing in the Output channels area?
Try them all until you get the configuration you're looking for.