The ARSI indicator as discussed in "ARSI, The Asymmetrical RSI" by Sylvain Vervoort is available for download here.
Once downloaded, from within the NinjaTrader Control Center window, select the menu File > Utilities > Import NinjaScript and select the downloaded file. This indicator is for NinjaTrader Version 6.5 or greater.
You can review the indicator source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting Arsi.