Is Watchtower not available as an update to Version 3.8.21?
Unfortunately Watchtower is a new service available in the Security Audit feature of 1Password 4. If you're using OS X 8 (Mountain Lion) or higher, you might want to look at upgrading to 1Password 4. You can enter your existing license information on our upgrade page.
If you're not quite ready to upgrade yet, that's fine! You can still check the vulnerability of the sites that you use on https://watchtower.agilebits.com/.