Please, please, please never put anything in the default "AddOns" folder. While this is listed as the "Option #1" in the Armaholic FAQ How To Install AddOns
never do that. Never. Never. Never. :no
Always use Option #2
. Learn how folder structures and Windows shortcuts work. But more importantly, learn how to modify your ArmA client using mods and addons WITHOUT potentially screwing up the base install of the game (which is what Option #1 can possibly lead to).
Computer -> Local Disk -> Program Files -> Atari -> Arma -> @FolderYouMake -> AddOns -> (*.pbo files go here)
Then for your shortcut:
"C:\program files\atari\arma.exe" -mod=@FolderYouMake -nosplash