download from http://boscripts.blogspot.com/2009/09/notepad-mel-language.html
there has three files.
After I tried couple times we only need this file : " userDefineLang.xml"
selecting View->User Define Dialog
press import select " userDefineLang.xml"
you should find Mel in the very bottom of Language list.
if you want know more about User Defined Languages
go to help contents (Shift+F1)
Languages/User Defined Languages