Go to this thread to download the most current beta, which at the time of this writing is RC18:
Be aware that while the beta versions do not require a license, they are set to expire on a certain date. L&L usually makes a new RC available before the old one expires, so make sure you backup your settings in case you need to manually uninstall an expired beta version and reinstall the newest. If you are using Scrivener every day, you can always use the Check for Updates menu item in the beta to upgrade to the next version.
Once the release version comes out, then the normal licensing code will start working again and the expiration process I just mentioned will no longer be active.
Also, use the Windows Beta forum for any bug reports, problems, etc. that you may encounter while using the beta – this allows the devs and other beta testers to see your posts more easily without beta-related information being splatted all over the forums used by folks who aren’t ready to use it.