Flash 11 isn't supported
Flash 11 is a new format that's still in beta. You can find information about it at http://labs.adobe.com/downloads/flashplatformruntimes_incubator.html.
Currently, Flash 11 SWF files can't be played in Flash Player 10, which is still the standard. We don't accept Flash 11 SWF files because most users can't yet view them. Additionally, we don't currently support Flash 10.2 SWF files, which can only be displayed in Flash Player 11.
When Flash 11 is out of beta and compatible players are widely used, we plan to support Flash 11 creatives. Until then, ask your developer to create Flash 10.1 or lower SWF files. Keep in mind that in the latest version of the Flash developer tool, Flash CS 5.5, Flash 10.2 is the default SWF file type. Make sure that your developer saves files as Flash 10.1 or lower, not the default Flash 10.2.