Ok, once you have the downloaded track on your desktop, open up the NL folder. There is a subfolder called Tracks. You can either create another folder in here to store downloaded tracks in, or you can just drop the track in the Tracks folder. Then, you can open up the simulator and select the track from there.
You can also just double click on the downloaded track to open it in the editor. The you can click the button that is black with red and yellow lines to open it up in the simulator.