Presumably you are using the app, which last time I checked couldn't support .rar files because it's a proprietary format. You can download and unpack the game manually from the site, which is inconvenient but should work. While you're at it, please petition the creator to repackage their game as a .zip file. :)
It would open it with realplayer, instead of Windows explorer like it normally would if it was a .zip file, I don't know why. Any help is appreciated.