Worksheet event code is stored on a worksheet module. To add it to your worksheet, do the following:

Copy the code
Select the worksheet in which you the code to run
Right click on the sheet tab and choose View Code, to open the Visual Basic Editor.
Where the cursor is flashing, choose Edit | Paste

Here is the code.
Private Sub Worksheet_Change(ByVal Target As Range)

    If Target.Count > 1 Then Exit Sub
    If Target.Column <> 1 Then Exit Sub

    If Target = "6111" Then
        With Target

            .ClearComments
            .AddComment
            .Comment.Visible = False
            .Comment.Text Text:="What the HECK!!"
            .Comment.Shape.TextFrame.AutoSize = True

        End With
    Else: Target.ClearComments
    End If

End Sub