Add the appropriate headers/merge fields by editing the letter content:
  1. Edit the Letter content
    1. For Appeal Mailings:
      1. Go to Marketing and Communications > Appeal Mailings
      2. Go to the Letters tab and click Edit
      3. Under Letter content, on the right click Edit
      4. Under write your letter's content, click Add or Edit
    2. For Acknowledgements:
      1. Go to Marketing and Communications > Acknowledgements
      2. Go to the Letters tab and click Edit
      3. Under Letter content, on the right click Edit
      4. Under write your letter's content, click Add or Edit
    3. For Reminders:
      1. Go to Marketing and Communications > Acknowledgements
      2. Go to the Letters tab and click Edit
      3. Under Letter content, on the right click Edit
      4. Under write your letter's content, click Add or Edit
  2. In the Merge fields drop-down, scroll to the bottom and click "More fields"
  3. Select the appropriate fields and drag them to the "Selected fields" box
  4. Click Save, click OK, then click Save
  5. Rerun the mailing and download the output again