[plt-scheme] problem with "mzscheme -r "

I've recently downloaded DrScheme, and have been making progress. However when I create an executable and then run it from the command line using the mzscheme -r command I get "not a command". I am using Mac OS X. There was no problems with the installation process. It may be a path issue, but I have tried to run the command from the directory that contains the mzscheme file, and it still get the same error i.e. its not a command. 

Thanks 

Bill