Applies To: Any add-in
Symptoms: The add-in does not appear in Outlook, is not visible in the Outlook window, or Outlook reports "Outlook experienced a serious problem with the 'sperry software add-ins engine' add-in."
Cause: Mismatched "bitness"
Resolution:
Many people make the mistake of assuming that Outlook (and really all of Microsoft Office) is 64-bit if the operating system is 64-bit.  However 32-bit applications run perfectly well under 64-bit operating systems and for a time, Microsoft was recommending 32-bit Office even if the machines were capable of running 64-bit Office.  If the add-in that was downloaded was for 64-bit Outlook, but 32-bit Outlook is installed (or vice versa), then the add-in will not show up.

To fix this, determine what version of Outlook is installed by starting Outlook, then going to File...Help.  The version number is displayed on the right and to the right of that the words "32-bit" or "64-bit" will appear.  Then determine the "bitness" of the add-in that was installed by going to Control Panel...Programs and Features.  Find Sperry Software in the list and if you see "(x64)" in the name then it is a 64-bit installer.  If the Outlook bitness and the add-in bitness do not match, then the add-in will not show up or work in Outlook.  Uninstall it, then  download the correct one.

Note: If you're still not sure which version of Outlook you have, try the article: "How to tell which version of Outlook is installed"