I created a simple script and attempted to run it. All that the scrip contained was "ls -l".
At first I received the message "ksh: run_dir: not found"
I then tried typing "csh run_dir" This time the script worked.
typing echo $SHELL produced /bin/ksh

I would like to understand why this happened.

thank you for your help

Your PATH variable may not be properly set up. If your working directory is /home/susan - set this in your '.profile' file,
Then run the profile (by giving '. .profile') and try executing the script again.

or

thank you - that worked

but what does ./ mean?

'.' - means the current directory. Since the script is in the current directory, ./run_dir amounts to giving the complete path of the script to be run.