If using Content Manager simply drag the downloaded file on to the Content Manager window.
Pre-requisite: install 7-zip (or another 7z archive utility if not available in Windows).
Locate the Assetto Corsa directory using Steam.
Right-click Assetto Corsa in the Library, then go to Manage -> Browse local files:
Navigate to the content\tracks\ directory in the explorer window that appears:
Right-click the address bar and copy the path to the tracks directory:
Open the downloaded track file with 7-zip. Click Extract and paste the path into 7-zip: