The Adjusted Rate of Change indicator, as discussed in "Two-Point Problem Of The Rate Of Change" by Martti Luoma, 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.
You can review the indicator's source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting AROC indicator.