That file is located at the root folder of Joomla installation. You don't need to use an text editor to do the Rename action. You can just rename it in the file explorer (My Computer) . However, maybe your Windows OS has hide the extension name (.txt) , so you need to disable that "hide file extension name" from Folder Options of your Windows.
If you feel very hard to do these, please find some time we are both online, I will try to operate on your computer via the "desktop share" feature of Skype. Maybe tomorrow ?
EasyCalcCheck Plus - Warning: The form was not found, because the template uses overrides. Please adjust the search string for the form in the file easycalccheckplus.php or delete the overrides in templates/yourtemplate/html!