You may see a warning saying: "This file is not commonly downloaded and could harm your computer." Don't worry.
We assure you that it's safe. The installer contains some custom configuration from your RescueTime account, and this causes each one of our installers to be slightly different from one another. This trips one of Microsoft's security checks, and causes the warning message to appear.
Just run the installer and follow the setup instructions.
Once installed, you will be automatically redirected back here to configure your account.