You can install it directly from the Moodle plugins directory through the Site administration > Plugins > Install plugins menu. Just browse the directory and select install. (Your Moodle site need to be registered.)
You can download it from the link below on the Moodle plugins webpage. After downloading it, unzip the content in the /mod folder of the Moodle directory.
Alternatively, you can download it from the GitHub repository.
Log in to your Moodle webpage as an administrator and visit the administration page. All the plugins to be updated or installed will appear, including the Treasure Hunt module. Click on the Update Moodle database button in order to proceed with the installation.