Reply to Recording a mix on the S4

Really basic question but I don't know how to record a mix from the S4?

There is the recording button at the top of the hardware next to the FX2 units but it doesn't seem to do anything when I press it.

I'm sure the S4 has the capability to record (Traktor even sets up a "recorded mixes" folder when installing the software) but I can't work it out.

Any tips gratefully received!

Thanks