How do you enable MouseKeys without using the mouse?

Recently, I had to shut down a Mac with no mouse and only a PC keyboard with no Eject key. (Incidentally, there are several solutions to that puzzle, and the one I settled on was to use the Terminal.) But as I first approached the problem, I thought: There is some universal access feature there to help if one can't use the mouse, isn't there? Yes: MouseKeys. However, this nice page with the keyboard shortcuts for MouseKeys in Lion suggests that you have to enable a checkbox in a System Preferences pane before you can use it. So, how do you enable MouseKeys without using a mouse?