I assume you are using Flash Builder as your IDE (could you let us know what IDE / version do you use? ). Flash Builder is an IDE based on Eclipse. The error you encountered seams to be a plugin installation issue and this is why I've suggested to try UML4AS on a new installation of your IDE.
It is possible to install UML4AS on Eclipse. To do so, you can download Eclipse from http://www.eclipse.org/downloads/packages/release/indigo/sr2
and just unzip it. Then you can run UML4AS installer and select the eclipse folder during installation. Also, note you can link Flash Builder to Eclipse by using Adobe Flash Builder 4.6 Plug-in Utility.exe, found in Flex Builder install folder (for FB 4.6).