The extension does not contain valid signature.
The extension will not be installed.
If you’ve run into this error message while trying to install a Photoshop CS5 / CS5.5 extension on a Mac (Lion – 10.7.2), be aware that this is an Adobe Extension Manager CS5 / CS5.5 known bug. Adobe will soon release an update, in the meantime they’ve provided a temporary fix. A full thread in Adobe Forum is available as well.
By the way, an interesting MAX 2011 archive video called How to Develop and Monetize your Creative Suite Extensions has been published, with sneak peeks of the CS SDKs future by product manager Gabriel Tavridis – get a look at it!