How do you turn on FN key on a toshiba?

The "Fn" key is a modifier key, much like "Shift." By itself, it does nothing. Its purpose is to use an alternative function, such as volume control or to switch to another monitor, that has been...

just press the Num Lock key

press fn and hold then reach for Num Lock, press them together and release

press fn and scroll.

Try pressing the 'Number Lock' key once to turn if off and see if it helps. Tapping the 'Fn' key five times may also work, however the first option worked for me.