See if this information helps: ProSeries User Guide, Chapter 18, Inserting codes and prewritten paragraphs into letters (intuit.com). You should be able to double click on the custom paragraph you want included.
I am able to double click and I see it the code, etc. entered. Then I preview the letter and its not there. This program is trash for letters and instructions. I shouldn't have to do anything. If the return is present in the system the instructions should generate automatically.