1. Check the Petroleum Office license status under Petroleum Office Tab => Settings button on ribbon => Licensing section. If license is expired of trial expired, functions from add-in would not be loaded.
2. Check if Petroleum Office is active in Excel add-in list. There must be 2 Petroleum Office add-ins - one is Excel add-in, another is COM add-in, both must be active.
If Excel Add-in is not active, try to activate in - Go... button at with Manage Excel Add-ins.
3. In rare situation the Petroleum Office Excel Add-in is not in the list. You need to add it manually. Notice the location of COM Add-in, both Excel Add-in and COM Add-in packed within the same .dll file. Click Go... with Manage Excel Add-ins. And then Browse...
Select All Files (*.*) from drop-down button next to File name, navigate and select the same file as COM add-in.