You can run the Venapp you are editing anytime by using the File>Run Venapp command. The Status Bar at the bottom also contains a shortcut for doing this.
The left hand portion of this works identically to the regular sketch editor, allowing you to navigate the different screens in the Venapp just as you would navigate different views in a model. The remainder of the Status Bar is Venapp Specific.
Run Venapp is a shortcut to the File>Run Venapp command and it launches the Venapp opening on the first screen. This will allow you to test the behavior of the entire Venapp.
Screen Definition is a shortcut to the View>Screen Definition command. This opens the Screen Definition dialog described above.