You are unable to run the command as the system doesn't know where to look for the mt_prep_gamma script file.
Open a Terminal window.
To add the directory of the mt_prep_gamma script (in this case the StaMPS bin directory) to the path list type:
You should now be able to execute the command.
To check the updated environment variable path list type:
The newly added path should appear in the list.
To run the command and avoid any errors you can format it as follows:
mt_prep_gamma [yyyy/mm/dd - of master scene] [output_directory] 0.4 2 2 50 200
(given that you want to leave the default prep options)