Go to the website you are downloading the Addon (In this case i am using ArmaHolic) and click Download

Go to your Downloads and drag the file to your Desktop

Right click on the WinRar File and select "Extract to Desktop"

Once you have extracted the WinRar file you should see either files or a folder on your desktop, Open the Folder and Locate the highlighted file types.

All the .PBO & .Bisign Files will go into the Addons folder (Required Folders Located in The Arma 2 File Location)

If some Addons come with a .BIKEY file then you have to place the .BIKEY files into the Keys Folder.

That is all that you need to know about adding Addons to your Arma 2, If you have any questions then comment and i will try and help you with what ever issue you have :)