Select the Add-in option tab. This is where the 'Save This Document to the Action' button is located.
This function only works when sending a letter from a record. It is not available in a mail merge.
 

Alternative solutions:
 

  1. Save the letter as an action
  2. Set up the merge letter in Configuration
  3. Create a constituent letter or a gift letter directly from the record
 
OR
  1. Globally add a constituent appeal to the recipients of the mail merge letter

OR
  1. Globally add an action to the recipients of the mail merge letter. The action information is the same for every constituent, and each recipient's letter is not saved as part of the action. The action's purpose is to indicate the constituent received a letter.