If you see a message about Adobe Flash Player stopping a potentially unsafe operation when working with Flash-based videos, you may need to modify your Flash security settings. To do that, go to the Flash Player Settings web site.

On the Global Security Settings panel, click the Edit Locations pop-up menu and select Add Location, then click Browse for Files on the next dialog that appears. In the OS X Open dialog that appears next, navigate to your copy of Usher (typically in the Applications folder), select it, then click Open.

You should now see Usher listed in the "Always trust files in these locations" box, and the error message should be gone for good.