Different Between TGB1.2 and TGB1.5 Gui Builder. Need Help

How do I use the Gui Builder in TGB1.5, I feel that different from TGB1.2 where last time I can place the textbox, button..etc to the game area easily by access the Gui Builder then select the "mainScreenGui" in TGB1.2. But I could not see "mainScreenGui" in TGB1.5. Please guide. Thanks