like you get a file say " Mod3.rar" or "Mod3.zip", if its a RAR file, download winRAR, just search for it under www.download.com.
then right click it and unzip/rar. like "unrar here", you'll see another folder, copy or move that in to C:\program files\lucasarts\battlefront\data\add on, but make sure the folder is like "mod3", and then directly in that there is a data folder and and addme.script, not like "My Third Mod > mod3 >addme.script, data", and like bulldog said make sure you have the latest patch, where you can get at www.lucasfiles.com
(download it, unzip and double click the installer. it will do the rest.