The PBO file should be put in to play it from Singleplayer Missions:

C:\Program Files\Steam\steamapps\common\Arma 2\Missions
or
C:\Program Files\Bohemia Interactive\Arma 2\Missions


The SOM_Transport_Demo.Chernarus folder should be put in to access it from the Editor:

C:\Documents and Settings\\My Documents\ArmA 2 Other Profiles\\missions