How do you arm the speed brakes in the PMDG 737, so when it lands the panels automaticaly pop out of the top of the wing ?

You need to bring the speed brake lever to the ARM position. You can do it with your computer mouse, or use Shiift+# (if I remember correctly), or use (if available) a lever on your hardware.

 

Hope, that answers your question.

As  the above has said  thought it was  a trick question    (I got mine set  to  r shift + / )