The start button? - If you mean the on-screen start button it has no function in Amiga emulation, only A and B button are usable. Some games from Amiga have been started by clicking on the mouse button. To do so go to settings and change touchscreen function from "Pad X" to "Mouse X". Usually the joystick has been connected to the second port while in the emumaster it is connected to first at the start point. You can change it from settings too.