I downloaded and Installed NinjaTrader_Paul's NT8MirrorBar add-on bar type but did not show on the list of bar types on the chart.
It appears that it has a conflicting bar type number as another custom bar type number (MirrorBars - 666 vs AlanPercentBarType - 666).
Obviously, to resolve this conflict one of these bar types must be edited and use a new number by either Paul or AlanP.
Cheers!