Interactive systems usability could be enhanced if users learn all the knowledge - the usabilty model - that capability them in applying designer`s potential solutions to domain tasks. The Semiotic Engineering approach perceives interactive systems as metacommunication artifacts that send a message from designer to users whose expression is the lower-level messages exchanged between user and system and whose content is the usability model. Starting from this perspective and based on semiotic theory concepts from Charles S. Peirce and Umberto Eco we present conceptual models to the interface as the expression and to the usability model as the content of designer`s message. We also develop a semiotic system to support user interface design. The system is composed by a specification language and rules that maps specified messagens to user interface widgets. Our emphasis here is not in aesthetics aspects of user interfaces, but in the interactive and performing nature of the interface message as it is carried throughout the computational medium.