This post accompanies setting up a userSetup MEL file, which is a script file that tells Maya to run the contained code when starting up. If you've made one on a Windows OS and it's not doing anything at all, this is for you.
By default Windows hides the extensions for known file types from the user (basically anything that a program on the system recognizes and Maya does recognize MEL files). Most likely to deter someone from renaming a picture from their vacation and accidentally deleting the ".jpg" part of the file name then getting scared and frustrated and eventually draining the life from some poor soul on a tech support line.
If you've made your userSetup file using Windows notepad then it will have a .txt extension that you will not see. So what you think is your MEL file is actually this:
To make Windows show you file extensions, go to Start > Control Panel > Folder Options (or "File Explorer Options" if you're on Windows 10). In the "View" tab, uncheck "Hide extensions for known file types" and click "OK".
Now you can rename your userSetup.mel.text to userSetup.mel. And you're done!
Edit:12/30/2015 - Updated for Windows 10 UI changes.