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.
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.
Select All Files (.) from drop-down button next to File name, navigate and select the same file as COM add-in.