(* excel_addcomment.applescript *)

tell application "Microsoft Excel"
    tell range "A10" of active sheet
        set cmt to its Excel comment -- no error if nothing (dummy comment)
        set vis to visible of cmt --get any property , returns missing value if empty
        if vis is missing value then
            set cmt to add comment
        end if
    end tell

    tell cmt
        Excel comment text text "My comment" --overwrites, but omit 'over write'!
        set visible to true
        select its shape object -- to allow editing
    end tell
end tell