If you download a .zip or .rar file there should be a folder, usually with a name of only 4 characters e.g. nab5, dra3, but regardless it should contain a folder 'Data' as well as 'addme.script'. Simply copy this folder (nab5 etc., not Data) into GameData/Addon and it should work fine. Eddie's maps make this a lot simpler by automatically installing the map into the correct folder so you should be able to play those with no trouble.
One more thing- do you have the 1.2 patch because i think if you don't you won't be able to use maps where the folder name is more than 4 characters