You can add this to the config file on the smoothie (don't forget to reset the board after the modification!)
# Switch module for fan2 control
switch.fan2.enable true # Enable this module
switch.fan2.input_on_command M108 # This switch is turned on when M106 is sent
switch.fan2.input_off_command M109 # This switch is turned off when M107 is sent
switch.fan2.output_pin 0.24 # This pin is turned on when this switch is turned on, and vice-versa
switch.fan2.output_type digital # PWM output settable with S parameter in the input_on_comand
And then add M108 and M109 to the start/stop gcode sections in LW4. Or if you want it to always go on/off together with the compressor, change M108 and M109 in the above section to M106 and M107 respectively!...
Then hook up the driver side of an SSR to pin 0.24 of the smoothie and the switching side to the Bofa AD250 power supply...
I added IEC panel sockets to the back of the FABkit for power in and out for that purpose! (See also the abovementioned Google Plus thread comments for that!!!)