Super Mario Bros. Crossover was designed to be played with a gamepad. Yes, it can still be played with a keyboard, but I highly recommend using a gamepad of some sort. It is difficult to relive the classic Nintendo experience without one.
The platform that the game runs on, Adobe Flash, does not support gamepads directly. However, you can bypass this limitation by emulating keyboard input through the gamepad. In other words, whenever you press a button on the gamepad, the game will think you are pressing a key on the keyboard. If you have a PC, there is some great free software called JoyToKey that does this for you. If you’re on a Mac, you can use the demo version of ControllerMate. If ControllerMate isn’t your thing and you have a Wii remote, there is a program called Darwiin Remote that allows you to use your Wiimote as a controller on a Mac. On Linux, you can use qjoypad. You can set your buttons in the game, so it doesn’t matter what keys you match the buttons to. If you don’t have a gamepad for your PC, I highly recommend using an Xbox 360 controller. I believe there is a way to use a PS3 controller as well.
Full Screen Mode
Another limitation of Flash is that it doesn’t allow keyboard input while in full screen mode. So if you want to play in full screen, you need some kind of workaround. On a mac, you can set up the “Universal Access” options in “System Preferences” to zoom the screen. That’s very easy to use. I’m sure there’s something similar on a PC. Or you can always just change your display resolution to something like 800×600 or 640×480 while you’re playing and change it back when you’re done.
See, with a few workarounds you can pretend you’re playing Nintendo.