Hi hanko
The non-pro version isn't an add-on. It's just a part of the ATUM free plugin.
Please, make sure you have the Purchase Orders PRO plugin active on your plugins page.
And if you are still having problems after the plugin is active, please do open a private ticket and we'll try to help you out.
Downgrading is never recommended because it could cause more problems than benefits...
Best Regards,
Salva.