Users may add letters to the Letters tab of existing revenue records by selecting the desired letter from a drop-down list of letters. This occurs when the output type is set as Export definition for the specific letter. Revenue letter does not appear in the drop down selection when Adding a Revenue letter on the Letters tab if the Output type is set as Export definition and the gift already exists.
However, users are able to view and select the letter when adding a new payment, on the Transaction details tab of the Add form.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.