I'm confused by your post. When you select the output filename, you should not be able to select the input filename. It is unclear from your word "addended" what you are doing.
Something is not right with your installation of Keyrite. Please uninstall it and install it again. The fact that the Check MTU button does not work further indicates the problem is related to the installation. Also, please post the answers to these questions as a Reply to this thread.
What version of Windows are you running?
Was it a clean new install, or did you upgrade from a prior Windows version?
If from a prior verison, which one?
What directory did you install Keyrite into?