Ben, while you have it open in Screen 4, click on the button to open the button set up box. In the middle of the page you have options of assigning it a system function, G-code, OEM code or VB script edited in Mach. Choose this one and save the screen. Open mach 3 with the screen that has that button in it. Go to top, operator, edit button script. The button should be flashing now. Click on it and add your VB and save. You should be set now. Might want to save current layout too in view. Sometimes the VB has a hard time sticking.
Brett