Hi Series9,
I have not used a mac before but I have OpenRocket on a windows computer, it looks like you are try to save to a file that has been moved. Were you editing a previous design? If so, did you change the file structure for that save file, while you were editing it? I have had an issue like I just described, the way you fix it is to find that file in it's new location, then overwrite it with your edited file. I hope I understood your problem correctly, OpenRocket is a great program to use, but it stinks to have problems with it.
BuiltFromTrash