I had the same question. However, after reading the specs on the magic keyboard -- for my iPad Air it's based on the light conditions around you. So, I went into a dark room and it automatically turned on. From there, you can go in the settings and adjust the lighting. There's no way to turn the backlit on and off manually for me. Hope that helps.
"Like an iPad, the brightness of the Magic Keyboard automatically adjusts based on the light conditions around you. If you want to manually adjust the brightness of the keys on your Magic Keyboard, go to Settings > General > Keyboard > Hardware Keyboard."