I tried adding the code 90756 using the "define codes" section, after adding the code I am not getting an error message. It saves the vaccine. When I select the vaccine to be given to a patient however no CPT code shows, only the given name of the immunization. When you add the new CPT code in order sets, you can lookup the CPT code(lookup button next to field to put in CPT code). When you click on lookup button, only blank lines are returned. This would mean that the code is not in the database,
In this case you will have to follow Mark's suggestions above to add the code to the database.
By the way I am using version 10.0