Maybe the code from Debra Dalgleish's site will help:
http://contextures.com/xlcomments03.html#Plain

Pietro wrote:

Hi all,
I have a button on my excel sheet that i want to assign to add comment:so i
want users to select a certain cell and click this button to add a comment to
this cell,in case there's already a comment,i need that it's ready to be
edited..does this make sense?

--

Dave Peterson