I'm not quite sure what you are doing but I had to change from making/ adding to the ACAD.MNU file to my own custom one.
Before about 2008 you got the file ACAD.MNU and were able to change it - but now they only give you the compiled ACAD.CUIX. So I left that alone and made a MYCUSTOM.MNU file and added it in using MENULOAD. This creates a MYCUSTOM.CUIX file which Acad uses and bolts it onto the end of the normal menu. Over the years there have been many different ways of making the menubar but it seems to have settled down now to this.
Hope this helps