If you're not using a mod manager, just open the 7z file, double click "MODS" under your winrar or 7z program, then again "Watkins Glen", and you'll see in there three folders: "content", "extension" and "system". Just select them and drag them all into the root of your assettocorsa folder, and agree to overwrite contents on folders.
After this is done, confirm that on assettocorsa\content\tracks a lilski_watkins_glen folder will exist (so as the other shader files that go into assettocorsa\system\shaders, which are obligatory for vanilla AC, or the \extension stuff if you use Custom Shaders Patch).
If using a paid version of CM, just drag the "MODS" folder inside the 7z file to your assettocorsa\ root, agree to the overwrites, and then in CM go to content>mods and activate it (Watkins Glen will become listed and manageable). There's also the drag n drop option (the whole 7z file) directly into the cm window, and then check the menu icon (top right) that becomes green and agree to install everything.
But first of all, try to find where you've previously uncompressed stuff for your failed install and delete it. Specially the lilski_watkins_glen folder if it's anywhere else other than directly in assettocorsa\content\tracks. Also, check if you accidentally created a "MODS" folder or a "Watkins Glen" folder under assettocorsa\content\tracks by mistake, and delete it.