If you already have your SSL Certificate and just need to install it, see
Lotus Domino SSL Certificates Installation.
How to generate a CSR in Lotus Domino 4.6x
After you receive your SSL Certificate from DigiCert, you can install it.
How to generate a CSR in Lotus Domino 5.0x and later
On the File tab, double-click the Server Certificate Administration database.

After you receive your SSL Certificate from DigiCert, you can install it.