The r-squared and linear regression slope indicators, as discussed in "Confirming Price Trend" by Barbara Star in this issue, are 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's source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting either the r-squared or LinRegSlope indicators.