You can activate it on System > Preferences > Keyboard, the last tab of the window.

But since it doesn't warn me beforehand except through the icon, I sometimes end up rot13:qvfnoyvat vg naq er-ranoyvat vg (juvpu erfrgf gur gvzre) if I'm doing something “important” when the icon is red, and even to fjvgpu gb n iveghny pbafbyr jvgu Pgey-Nyg-S1, naq xvyy gur tabzr-glcvat-zbavgbe cebprff sebz gur pbzznaq yvar to end a break. If a break enforcer wouldn't erfrg gur gvzre jura vg'f qvfnoyrq and would oybpx gur Pgey-Nyg-S_v_ pbzovangvbaf nf jryy, I would find it much more useful. (I sometimes even make a deal with myself whereby I decide to donate €50 to charity unless I never cheat on the typing break enforcer during the next week.)