It's mentioned on the official website that BetterStabilizer is a plug-in that works for Final Cut Pro X and Adobe Premiere Pro. After you install the correct version and have the correct version of the apps installed as well, it should be available from within the main interface.
I suggest checking the manual of the plugin after you acquire it to see how you can access it. As far as I know, you need to load some content for certain menus to be available.
Also, visit the following Tutorials page to get an overview about the usage of the plugin.