I have never heard of .cse for a PCStitch file. It has to be from BEFORE PCStitch 6 (current is 11). PCStitch 7 was somewhat recent when I first started using PCStitch and that was back in 2004! That puts your file possibly into DOS times.
It may just be too old.
If the PCStitch Pattern Viewer is still available, you might be able to see the file there. That hasn't been updated since PCStitch 8 as far as I know so it MIGHT handle that file.
My only suggestion is to contact Mark and ask if you can send him the file via email and he convert it to .pat and send it back. That's the only thing I can think of to handle a file THAT old.