Create a textbox named txtChecked, size it, set the font size to 18 (or whatever size you want), set Enabled and Locked properties to Yes, and add this code to the On Click event:
If Me![txtChecked].Value = "X" Then
Me![txtChecked].Value = ""
ElseIf Me![txtChecked].Value = "" Then
Me![txtChecked].Value = "X"
End If
Hope this helps