Is it to the right format of your card? I know there is a website for converting .sav files, just getting the link now.
EDIT: Here it is: link. I had to dig through my bookmarks to find it xD. Also when I first got Dark Souls it wouldn't start the game because for some reason it wouldn't create a save file. My advice is to goto the download section and get one but you already have one so just convert it and all will be well.
Edit: Oh whoops, my bad, I don't have time to check what type of .sav it is, but most likely it will be a no$gba save. If that doesn't work, just go through the other .sav's until it works out. There aren't that many, and converting doesn't take very long. G'luck mate.