Warning Msg Box For Missing Data On A Form.

I would like to know how to display a msg box for missing or duplicate information when entering data into a form. My database keeps track of call accounting codes and the code is the primary key index field. I use macro's for all of my automated tasks such as finding codes, filtering for available codes and such. What I want to do is to display a simple message when adding a new one to the database if they forgot to enter the code and if they did enter a code if they entered a duplicate. The default error meesage tells them there is a problem but they won't know how to fix it. I know I should use the Before Update and After Update but I am not good at writing the vb code. I found some code in one of the the threads and copied it and changed the field names but it failed. I need a simple message like "You did not enter the Fac code" and "The Fac code you entered is already in the database please check your information."
The field name of my primary key is "Fac"
Thanks in advance for anyone that could help me with this.