You need to make sure that the executor is marked as "proving", and that they have an entry in the address book.
Check this by going to "File...Open...Browse the full list of matters" and selecting the "executors" page at the bottom. Right-click on the executor in question and select "Address book entry" - you'll be taken to their address book record (or prompted to add one if it's missing). The name and address data will be sent into the document the next time you tell the system to fill the document.