In the Zen Cart administrative interface, what version is shown as installed when you go to "Configuration" -> "Edit Orders"?

If it does not list a version, or the version listed does not match the version of Edit Orders you believe is installed... I would suspect something is not right with your installation of Edit Orders (or the installation is incomplete). I would suggest downloading a clean copy of Edit Orders, applying any needed patches from the Edit Orders support thread, and re-installing Edit Orders.

It would also be a good idea to check your "cache" and "logs" folder for any debug files. If running Zen Cart > 1.5.1, be aware you will need to manually merge the required changes to core files (do not blindly overwrite these files).