Log in to a test or real registry web interface and add IP address of your server. Note that server host IP address and your website IP address may differ. Contact your server administrator to obtain correct IP address.
EPP host for testing registry server is: https://devepp.ics.forth.gr:700/epp/proxy
For production registry: https://regepp.ics.forth.gr:700/epp/proxy
It is usually the same as EPP login.
Your firewall must be configured to accept connections from/to TCP ports 700(EPP) and 800(whois).
There is no need to do anything with cerificates, as test registry certificate is already inplace.