I assume you mean this one: https://anagan79.itch.io/quest-games-optimizer
This project needs to be purchased first, if you did not obtain a download key from the developer, nor was it gifted to you by another user.
You can do so on the bottom of the page ('Purchase' -> 'Buy now' button). If you already own/have purchased it, you can either download it from the top of the page afterwards, or by going into your library ('Down Arrow' on the top right next to your user name -> 'My library') and looking into 'Things you own'.
You may also want to take a look at the page description first before purchasing/downloading, as it seems to have a comprehensive documentation and FAQ for users to work with.