lets say here is your game path; C:\Program Files\Bohemia Interactive\ArmA 2
Now inside the ArmA 2 folder, lets say your mod is called "JTD" your mod folder should look like this; @JTD then inside that folder should be another folder called Addons, place the JTD mod addons inside there. (and if there is a "KEY" addon file it goes inside the KEYS folder
Now use the A2 launcher, it will detect the new mod folder automatically. Best tool, makes it all very sexii
For further info check the FAQ page
edit; how do i set the link over the word "FAQ"??