If you have installed the add-in, following the instructions here and the add-in is still not there, please try closing outlook and restarting it again. With some versions of Outlook, a good old restart seems to do the trick.
If the add-in is still not showing up in your calendar, please let us know your Outlook Version. You can access this information with the following steps:
Go to File
Click "Office Account"
Share the information in "About Outlook" section
Once we have this information we should be able to help you.