I believe XenStaff is also a Xenforo 1 add-on, Xenforo 2.2.3 is the latest version of the software and XenStaff is not compatible with this version, using an old version of Xenforo is never advised.
Thanks for your interest in that add-on (I am surprised how many people are still interested in XenStaff after all this time).
XenStaff was free, and is no longer available. It is not compatible with XF 2+. It was re-written and is now called Highlight groups, which is not free (7.95 CAD).