you are correct, i meant Space (not Backspace), i blame my limited english for that
). However, i got it somewhat working now, i mapped Space to a Button on my Joy and i can change the ammo now. However its still not possible to use the keyboard (Mouse is working) which makes playing Faery Tale Adventure impossible (you need to enter the correct passwords to continue to the game) . Here are some screenshots from the Input Tab (maybe that helps a bit).