'Code to be placed in the worksheet(s) you want to use this in
Option Explicit 
Public oldRange As Range 
 
Private Sub Worksheet_SelectionChange(ByVal Target As Excel.Range) 
    On Error Resume Next 
    Dim rng As Range 
    Set rng = Target(1, 1) 
     
    oldRange.Comment.Visible = False 
     
    With rng 
        If Not .Comment Is Nothing Then 
            If .Comment.Visible = False Then 
                .Comment.Visible = True 
            Else 
                .Comment.Visible = False 
            End If 
        End If 
    End With 
     
    Set oldRange = Target(1, 1) 
End Sub 
 
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Excel.Range, Cancel As Boolean) 
    On Error Resume Next 
    Dim cmtText As String 
    Dim inputText As String 
     
    If Target.Comment Is Nothing Then 
        cmtText = InputBox("Enter info:", "Comment Info") 
        If cmtText = "" Then Exit Sub 
        Target.AddComment Text:=cmtText 
        Target.Comment.Visible = True 
        Target.Comment.Shape.TextFrame.AutoSize = True 'Remove if you want to size it yourself
    Else 
        If Target.Comment.Text <> "" Then 
            inputText = InputBox("Enter info:", "Comment Info") 
            If inputText = "" Then Exit Sub 
            cmtText = Target.Comment.Text & Chr(10) & inputText 
        Else 
            cmtText = InputBox("Enter info:", "Comment Info") 
        End If 
        Target.ClearComments 
        Target.AddComment Text:=cmtText 
        Target.Comment.Visible = True 
        Target.Comment.Shape.TextFrame.AutoSize = True 'Remove if you want to size it yourself
    End If 
     
    Cancel = True 'Remove this if you want to enter text in the cell after you add the comment
End Sub