Check the constituent record in The Raiser's Edge that you wish to honor/memorialize.  The following must be true of the constituent record in for the specific tribute to populate in Blackbaud NetCommunity
  1. The constituent may not be marked as marked as "Inactive" nor marked as "No Valid Address" on Bio 1
  2. The constituent to be honored must be flagged as an honor/memorial on the Honor/Memorial tab of the constituent record
  3. There must be at least 1 tribute listed 
  4. On the tribute listed, there must be an Acknowledgee set

If you attempt to change these items on the constituent record while you have the donation form design in process, the drop-down will still not populate.  You must cancel your changes of the donation form, save all changes on the constituent record, then begin editing your donation form or edit an existing one.