The Word file that you opened was selected in Windows Explorer and was appearing in the Windows preview. You then double-clicked the file to open it.

Background

The Windows preview starts Word, which prevents the Add-In from being correctly loaded.

Solution

  1. End Word.
  2. Open the Windows Task Manager by pressing CTRL+ALT+DELETE. Click More details if necessary. In Windows 7 enable the Show processes from all users checkbox. In the list of running processes, find Winword.exe or Microsoft Word. Select the process and click End task.
  3. Start Word by clicking the program icon.
  4. On the File menu, select Open and open the Word file.