Just go to the post that has the latest version (1.12.9) for the download links. Then look at the welcome mail you received for a hint to the password needed to open the .rar.
Well, the hint in the mail gives you everything you need. And once you figure it out, you will kick yourself because it was so obvious. I'm not comfortable giving any more hints, because the dev clearly doesn't want the password being mentioned here. If you absolutely can't figure it out, he mentioned the Discord as the place to ask.