a Plugin must be created with Flash CS3 or with the Commandline Flash compiler mxmlc,
to use the krpano interface the "krpano_as3_interface.as" must be imported direct in the Actionscript3 source code (.as) have a look here: AS3 Interface / Plugin Development
have also a look at the plugin souces (the .as files) in the "plugin-sources" subfolders in the download package,