Disabled/Missing Outlook Add-ins

If it's not in the Active Add-ins list, scroll down and see if it's in the Inactive list or the Disabled List. If it's in the Disabled list, In the same panel there is a drop down menu towards the bottom that allows you to choose Manage Disabled Items.

Chose that then hit "Go". The Add-in should be in that list. Select it and then select Enable. Restart Outlook and go back into Add-ins page to check if it it's showing as Enabled.