For your calculator to run as intended, it needs correct prerequisites. To check the prerequisites, go to the Application Manager (from the Transcript, choose Manage Applications from the Tools pull-down.) and select STCalculatorAppin the left pane. Next, select Explain Prerequisites from the Prerequisites menu. Information printed to the System Transcript window should indicate whether the application's prerequisites are correct. If the prerequisites need to be changed, refer to Creating the application STCalculatorApp and “Making your own applications: Creating applications: Checking prerequisites” in the Smalltalk User Guide.