The Formmail Script is provided via the Control Panel, full usage instructions can be found after installation.
Assuming that you have successfully installed the formmail script via the Control Panel, then all the files will be present and the file permissions are correct. Therefore the only other reason for a problem would be your form that is calling the script.
In your eXtend control panel when you installed the script you were given a link for “Usage Instruction” and example HTML code to use, please take a look at that and make sure yours is the same. You are able to make modifications to this to add your own fields, but the basic structure is there to follow.
In order for the form to fully display on a web page the page should be saved with a .cgi extension.