For windows : 
Go to your minecraft .jar (Go to run -> (type in) %appdata% -> .minecraft -> bin -> minecraft.jar)
Open that (and back up that and yours saves IF you want), extract the ModLoader files into there 
if you don't have them already, then extract the .class files from the NoteBlockGui Mod download into there. 
--------------DELETE THE META-INF FOLDER----------

I would suggest backing up your saves. You never know what could happen.