My guess is that global.input_pad is not initialized to a value before you try reading it - did you add it to init_input? You'll need to define a gamepad input for every action (U/D/L/R/A/B/C/ST/SL).
Viewing post in Yal's Monster Collector Engine comments
Add it in the same script (init to a null value), also set it whenever you get a "gamepad" Async:System event (which is triggered whenever a gamepad is connected or disconnected). It's probably easiest to handle these events by putting a persistent "gamepad connection controller"object in the first room, which is a child of parent_active (so it can't be deactivated) - then it sticks around throughout the entire game without you having to put one in every room manually.