Unfortunately, the SplashID plugin for Safari is not compatible with the Safari version (6.0) in Mac OS X 10.8 (Mountain Lion) at this time.
We are developing a new version of the plugin that will be compatible, and it will be available in a future update to SplashID.
In the meantime, you can still use the "Web Autofill" button found beneath the record details in the SplashID Desktop/Safe.
You will also want to uninstall the SplashID plugin from Safari to avoid getting the error message on launch.
This is done from: SplashID Safe ---> File menu ---> Plugin for Safari ---> Uninstall
NOTE: For SplashID Safe 7.x, please also see here: