For example lets say we wanted to download a Marine add-on.
Lets call it - marine1.pbo
once it's downloaded you find the add-on folder
THIS IS HOW YOU DO IT ON WINDOWS VISTA
Computer - Local Disk - Program Files - Bohemia Interactive - ArmA - add-ons
you then move it to a seperate place, For example
Documents - Downloads
and then move it in to the add-ons folder
That is a very Simple
Download, if you don't download it that way look in the read me manual, But sometimes there isn't a read me which is very annoying.