The add certificate name selection box for purchase orders only contains certificates for that vendor. Therefore if the certificate you need to add is not in the list of certificates, ensure that the certificate vendor is correct. You can change a certificates vendor in the certificates settings (Administration->Certificates->Certificates).
The reason the selection box only contains certificates for the vendor of the purchaser order is to reduce data entry errors. It also reduces the certificate list to make finding the certificates you need to add easier.
