When MouseKeys is turned on, the keys on the numeric pad have three different kinds of functions:
Keys 1-4 and 6-9 move the pointer around the screen.
The 5, + (plus), 0 (zero) and . (decimal point) keys simulate mouse button clicks and presses.
The /(slash), * (multiply), and - (minus) keys switch the keyboard mapping between various mouse buttons.
You can bring up a Status window to see which mouse keys are currently pressed.