Which version of Extension Manager and Dreamweaver have you installed? Have you moved or renamed the Dreamweaver folder? Do you have Windows or Mac? Are you in a corporate network? Have you updated EM and DW to the latest version?
Please note that Extension Manager only installs and manages extensions for the corresponding version of products. e.g., Extension Manager CC will not recognize Dreamweaver CS6, for which you should use Extension Manager CS6.
Please run Adobe ExtendScript Toolkit (CS6, CC or whatever version you have installed), input the following command