The certificate will be the same one that was emailed to you most recently and used to install Opus in the past. You can retrieve it automatically using your licence number with the Retrieve option in the Licence Manager in Opus.
Alternatively, you can have the email containing your certificate re-sent to you via Lost Registration Retrieval, assuming your email address is still the same.
You can also download the certificate from the My Account page if you know the email address and registration code.
For a step-by-step guide to that, please see Folder Formats: Quick Guide.