To install an indicator into NinjaTrader follow these steps:
In the NinjaTrader control panel click Tools -- Import -- NinjaScript Add-on
Then select the file. For example, if the file is on your downloads folder, find it there:
Select the file and complete installation.
Following successful installation, the indicator should now appear on your indicator list in NinjaTrader 8.
To find that, right click your NinjaTrader chart and select indicators to find the indicator on the list there: