Add a line of code:
Sub Highlight_WordN()
Dim oRng As Range
Dim oComment As Comment
  Set oRng = ActiveDocument.Range
  With oRng.Find
    Do While .Execute(FindText:="HAPPY")
      oRng.Paragraphs(1).Range.HighlightColorIndex = wdYellow
      Set oComment = ActiveDocument.Comments.Add(oRng.Paragraphs(1).Range, "Whatever text you want as the comment")
      oComment.Range.Style = "Body Text" 'Or whatever built-in or custom named style you want to apply.
      oRng.Collapse 0
    Loop
  End With
lbl_Exit:
  Set oRng = Nothing
  Set oComment = Nothing
  Exit Sub
End Sub