You need to put the zip in the “mods” folder located in your “user folder”. The link below explains how to access the user folder. The bottom link details manual installation of mods.
The challenge is accessed via free roam. Drop into the italy free roam, bring up the map, click on the green map pin over castelletto town, the menu on the left should show challenges available in casteletto. The challenge is a time trial challenge called targa3.
Apologies, this was the only way I was able to create the this track. If anyone knows how to add as a time trial any help would be great. Thanks.