Unchecking "Enable mouse Keys" using keyboard?

Ah. In that case, there is this, from the official apple support page.

When Mouse Keys is enabled with one of these keyboards, you can use the keys 7, 8, 9, U, I, O, J, K, L, M, and '.' (period) to control movement of the mouse pointer and mouse clicking, but other numbers, letters, and keys cannot be used. (Keyboards with no numeric keypad or Num Lock key include: Apple Wireless Keyboard (2007) or newer, Apple Keyboard (Early 2009), the built-in keyboard on MacBook (Late 2007) or newer, MacBook Air, or MacBook Pro (Early 2008) or newer.)

The following keys will continue to operate normally when Mouse Keys is enabled: Esc, Function keys, Eject, Delete, Tab, Shift, fn, Control, Option, Command, and the arrow keys.

A friendly link to the full article

Press right option key five times. That will disable Mouse Keys. Or you can press the M key to emulate a click.