Click the button on the right to install the addon. The listed permissions are required to block websites. No data collected is ever sent off your computer.
If nothing happens when you click install try copying and pasting about:addons in your address bar to make sure the addon is enabled.
If you get a connection error right click the install button and select "Save Link As..." Then, browse to the .xpi file and open it with Firefox.
After installing the addon, you should automatically be brought to another page to show you how to enable private window permissions.