I can't see why you'd want to add the same comment to all such cells but, if that's what you want, try:
Code:
Sub AddComments()
Dim ocel As Range
On Error Resume Next
For Each ocel In ActiveSheet.UsedRange.SpecialCells(xlCellTypeConstants)
ocel.Comment.Delete
ocel.AddComment "Comment Text"
Next
End Sub
Note that this code will replace any existing comments. If you don't want to do that, delete or comment-out the 'ocel.Comment.Delete' line.