toc There are two solutions:

LED Tactile Button:

solder Led Tactile Button on the smoothieboard
image led_tactile_button.png

Simple Button:

Wire a simple button on two pin
image simple_button.png

It is highly recommended the pause button be configured as a Kill switch instead

The pause mechanism causes the axis to stop immediately with no deceleration, and when restarted it continues
at the speed it was going with no acceleration. This will most likely cause the stepper motors to skip and thus lose position.

The pause button may be configured as a kill switch instead of pause, this will halt all operations and turn off heaters and pause the host. It can be cleared with a M999 or reset. to enable a kill button the following is added to the config… 

kill_button_enable                        true             # set to true to enable a kill button
kill_button_pin                              2.12             # kill button pin. default is same as pause button 2.12

When in the halted state (after kill button is pressed) the play led will flash rapidly, whereas when paused it flashes slowly.