You should first read the background for HOWTO Install OFP Addons (vtemod.com) and ArmA 1 Addons Installing which is basically general overview.
On top of that knowledge, you should also read the usage of PMC Editing Wiki: -profiles parameter (-mod= dir stuff) so you know how to load up the addon directories properly.
Your addon directory structure might look something like this:
<arma2_root>\x\ <arma2_root>\x\pmc\ <arma2_root>\x\pmc\addons\ <arma2_root>\x\vte\ <arma2_root>\x\vte\addons\
You will unpack the addon pbo files into the <...blabla>\addons\ directory.
When you have unpacked all the first, run ArmA 2 with a command line to point into these addon dirs, for example like this:
arma2.exe -mod=x\pmc;x\vte -nosplash -world=empty -profiles=d:\ArmA2
Take a note that ArmA 2 default EU/US (we think?) installer does not create <arma2_root>\Missions\ and <arma2_root>\Campaigns\ directories, you have to manually create these. You can place Campaigns into addon dir, but Missions dir for some reason since OFP times only work from arma2_root\Missions\ instead from a addon dir.
To install a PMC campaign (or any other for that matter), create your favourite addon dir, for simplicity we call it "pmc". So here we go, you should create:
<arma2_root>\x\
Then you should create the "pmc" dir:
<arma2_root>\x\pmc\
And finally the campaigns dir:
<arma2_root>\x\pmc\campaigns\
Now remember, missions do not work from <arma2_root>\x\pmc\missions\ unfortunately, you have to put them into <arma2_root>\missions\.
Now, but the downloaded campaign into the <arma2_root>\x\pmc\campaigns\ dir, then launch your ArmA 2 with these parameters:
arma2.exe -mod=x\pmc -nosplash -world=empty -profiles=d:\ArmA2
And the campaign will become available in the Campaigns menu. This way you have not altered or "messed up" the normal ArmA 2 directory structure at all. Very neat.
Singleplayer
Put the multiplayer mission pbo or directory (pbo is preferred) into your <arma2_root>\Missions\ directory. This Missions directory can contain sub directories as well, so you can categorize your missions, you can even add overview.html file to the sub dir with text and image (but that is beyond this guide).
Multiplayer
Put the multiplayer mission pbo into your <arma2_root>\MPMissions\ directory. On linux dedicated servers the mission file name has to be lower case (linux has tolower script to lower all file/directory names automatically).
On ArmA 2 Combined Operations v1.62 Missions and MPMissions directories do NOT work from a directory loaded with -mod= parameter.
Go back to PMC ArmA 2 Manuals home page.