Check the version of Outlook if on 32 bit. From Outlook click File
Select Office Account then click About Outlook.
The version will then be shown as highlighted below.
If it is on 32 bit, Go to this file path C:\ Program Files (x86) \ eXpd8\ Practpro, scroll down and double click RegUtil and click Register in Office then click OK
3. Close and re-open outlook and add ins should be back again.
Note: Expd8 add ins only work on 32 bit version of Outlook so if it is on 64 bit version. It will need to be downgraded to 32 bit for add ins to be installed.