Running the MCNPX input file: ex1, should give you a file called: wssa
Look for "Source Surface Write" in the manual.

If you rename this file to: rssa and run the script called: run.sh, that
should run McStas using as source the rssa file.

This will create a file called wssa, which you can rename to rssa and
run the MCNPX input file: ex1_read.

There are no tallies (detectors) in this file, this you will need to
define yourself. It is meant as an example to show you how the procedure
is done.

I hope it help -if not, please ask

PS: I realize that the renaming of files is annoying - but it was never
quite annoying enough for me to fix it...

best regards,
Esben