The Launch button should light up at the start of each new ball, when the ball is sitting in the launcher lane ready for action. Pressing the Launch button at this time just launches the ball, doesn't fire a weapon.
Once a ball is in play, the Launch button should light up if you have any weapon available, indicating that pressing the Launch button will fire the currently selected weapon. (If you're not in a mode/multiball, the Dashboard should also advise this on the LCD.) If you have two or more different kinds of weapons, the Extra Ball button should also light up, indicating that pressing that button will rotate which weapon is currently selected. If you have any weapons at all, one will always be "currently selected" to be fired with the Launch button, so if you fire it, the software automatically selects the next available weapon. It is possible to accumulate multiples of each weapon, so should you earn 6 weapons in a row without using any, you'll have two pistols plus one each of the other four weapons.
(I've got code sitting on my computer to show weapons status on the Airlock LCD during modes/multiballs, because you'll often want to use a weapon when those features are active, but currently it's difficult to figure out which weapon is ready to launch if the main Dashboard isn't showing. But because David Thiel is a freakin' genius, there's already a distinct sound effect played when each type of weapon is selected, so you actually can cycle the Extra Ball button and know which weapon is currently active through audio cues alone.)