To run FreeRouting a java runtime enviourment is needed next to the FreeRouting executable. Many system should already have it installed by default. If it is missing on your system, you can download it from the Oracle Homepage.
The easiest way to install the FreeRouting executable is to install the LayoutEditor package. It includes a compiled ready to use FreeRouting in the bin folder of the LayoutEditor package. To start the FreeRouting application just press on the freeRouting.jar file. Alternative you can download the FreeRouting source files from this web page and compile it with the Java Developer Tools.
Within the PCBnew tool plress the autorouter button . A dialog to the Freerouting tool will be opened. In this dialog you can export a Spectra Design File (.DSN). This is the input file you will need with FreeRouting. After opening FreeRouting open this file and preform the routing. When routing is finished store the result to a Spectra Session File (.SES). In the same KiCAD dialog this file with the routing results can be imported. The dialog also contains additonal help using FreeRouting.
There are some Youtube videos explaining it in detail: