I've added some buttons for my ATC to the 'tabPositionsExtens' tab (the first one shown on startup). It populates the button labels with the tool number. The script works, but when I start mach 4 the script is not called. I need to select a different tab, then go back go back to 'tabPositionsExtens' tab and everything is correct.

Is there a way to make mach 4 call the script when it starts up?

either place the code in the screen startup section or in the plc scripting.. that way it will run at startup.. you may need to change the code a bit so that it doesn't run every 'scan'... unless that doesn't affect the operation of Mach4.. it sounds as if you placed the code in the scripting for the Tab itself.