Ok, so the problem is that windows doesn't know where to find mzscheme.
To rectify the situation, you could either type in the full path to mzscheme instead of just it's name, or you could add it to your PATH variable (that's where windows looks to locate programs when you type them in on the command line)
Here's how to do that temporarily:
PATH=%PATH%;<insert mzscheme path here>
Again, that's PATH=%PATH%, followed by a semicolon and then the path to mzscheme. Unfortunately, this is a temporary fix that will only last as long as that command window.
To make it permanent: right click on "Computer" and go to properties. Somewhere there should be an Advanced Settings link, which should pop up a new window. Click the Environment Variables button, and edit the PATH variable (either for your user or the whole system). Add a semicolon, and then paste in the path to the mzscheme executable.
