Page 15, question 3.3 of the AN form:
This asks: Do you have any civil judgements against you or any civil penalty under the UK Immigration Acts?
You should provide details in the civil penalty in part 3.4 also. You must include the date in which you received this.
You may also want to include a cover letter with your application to explain when and why you received the PCN. This should not affect the application depending on the circumstances however it must be declared to the Home Office.