Perhaps its your level. Many members set the download restrictions to members of levels 2, 3 or sometimes higher. Add the thread(s) you would like to download to your favourites and come back when you level up to get it.
You need to build up your level by making posts on threads or making threads like this one. I am level one and there are a lot of paterns I cant get yet :(
I just came across my 1st "Sorry, only specified members can download attachments" as well. Now I gotta figure out how to level up 62 credits hopefully to gain access!