All addons and/or Mods should in the main ArmA 2 directory.
However, do not just transfer the addons into the ArmA 2/addons folder. BAD Idea!
What you need to do is create folders in the main directory of ArmA 2 and name it according to you.
So my directory would look like this;
Program Files\Bohemia Interactive\ArmA 2\@FunkySounds\AddOns (you will place the new addons here)
Hope that was clear, if not don't hesitate to PM me or other admins for assistance