How to Play Flash Games with a Gamepad
Although Super Mario Bros. Crossover can be played with a keyboard, it was designed to be played with a gamepad.
The following software can emulate a keyboard with a gamepad.
PC - JoyToKey
Linux - qjoypad
There are four buttons in the game, plus start. The following setup is recommended if you're using a Super Nintendo controller as a reference.
B - Jump (Z key)
Y - Attack (X key)
X - Special (C key)
Select - Select (A key)
Start - Start (Enter key)