Currently the Flash 9 beta installer doesn't install the plugin into the Flock browser. To get it working, simply copy C:\Program Files\Mozilla Firefox\plugins\NPSWF32.dll to C:\Program Files\Flock\flock\plugins\NPSWF32.dll.
Note: This is obviously not supported from Adobe. I urge you to back up your C:\Program Files\Flock\flock\plugins\NPSWF32.dll before copying over the new version, so you can revert back to it if this doesn't work for you.
Note: Flash plug-in 9 is NOT beta anymore and that’s for quite some time now. Flash 9 is final release, and that’s just for the player. Authoring tool Flash 8 Professional is still at version 8 (Version 9 beta is out) and they intentionally released Flash player 9 first this time. http://www.adobe.com/products/flashplayer