If you recently logged into Outlook and are missing one of your Outlook add-in buttons, do the following to reactivate it.
Steps to recover
1. Click on FILE tab (Outlook main screen, Top Left Corner)
2. Click on Options (left side corner of the window, on the blue strip, 2nd last option)
3. On the window which pops-up “titled – Outlook Options”, click on Add-Ins (left side corner of the window, 2nd last option)
4. It will give a list of Add-Ins, scroll on the bottom of the list – “Disabled Applications Add-Ins”
5. Click on the Add-in which has to be re-enabled.
6. At the bottom of the window is a drop-down menu and the button “GO”. Change the drop-down menu to 'Disabled Items' and then click GO.
7. Another window will pop-up labeled "Disabled Add-Ins”, the checkbox for the Add-In which has to be activated
8. Click on “Enable”
9. You may need to close out Outlook entirely and open it again before the Add-In will reappear.