Final Draft defaults to only looking for newer FDX files. It doesn't know you want it to look for the older Final Draft file format, FDR.
1. Open Final Draft;
2. Go to File Open;
3. At the bottom of the Open window, adjust the Files of Type dropdown menu to Final Draft 5-7 Documents (FDR); now all Final Draft 5-7 files will be visible;
4. Navigate to where your .FDR file is saved;
5. Double-click the file and it will open.
You will see a prompt advising you that you're about to open an older version of the file; dismiss this prompt.
Once this script has been saved in FD12 / 11 / 10 / 9, you do not have to follow the steps in this article again to re-open it.
You will have to follow the steps in this article for any scripts that have not yet been opened and saved in FD12 / 11 / 10 / 9.