are you asking about a note that you can refer to in the future?  use the little pencil or at the bottom of the client status there's a place to enter client notes ............. or 

are you asking about transmitting a note to the IRS with the return?  i think the 'additional information form' or 'miscellaneous statement' will work but someone else would have to weigh-in... there used to be something called preparer notes that I don't see/can't find anymore.