If your plugin-in disappears from Outlook, it is often because Outlook has disabled it for some reason.
Sometimes Outlook pops up a message when Outlook starts saying something like 'There was a problem with add-in XXXX last time Outlook ran. Do you want to disable it?" The default response is 'Yes' and the add-in will be disabled. Sometimes you may not even realise you clicked YES.
To check if your add-in is disabled, and re-enable do the following:
Outlook 2016
Select File >> Options >> Add-ins
At the bottom is a drop down marked 'Manage:' Select 'Disabled Items' in the dropdown and click 'Go'.
If you see your add-in in the list, highlight the add-in and click enable.
Outlook 2013
Select File / Options and click the add-ins tab.
At the bottom is a drop down marked 'Manage:' Select 'Disabled Items' in the dropdown and click 'Go'.
If you see your add-in in the list, highlight the add-in and click enable.
Outlook 2010:
Select File / Options and click the add-ins tab.
At the bottom is a drop down marked 'Manage:' Select 'Disabled Items' in the dropdown and click 'Go'.
If you see your add-in in the list, highlight the add-in and click enable.
Outlook 2007:
Select Help / disabled items.
If you see your add-in in the list, highlight the add-in and click enable.
Outlook 2003:
Select Help / About Microsoft Outlook ... and click disabled items.
If you see your add-in in the list, highlight the add-in and click enable.
If your add-in is not in the list, you can try to repair the original installation from the add/remove programs options in the control panel,or just re-install the plugin.