The drop down is gray in this case because the selected Tributee has been marked inactive or deceased. To add tribute letters to this tribute:
  1. Select the Constituents tab at the top of Altru.
  2. Click Tributes
  3. Find the tribute in question and click the hyper link in the "Tribute text" column. 
  4. In the Acknowledgee tab, click "edit acknowledgees" 
  5. From here add an acknowledgee/acknowledgee's and the Tribute letter/letters you want.