You will need to convert the key you created to PKCS12:

% openssl pkcs12 -export -out mynick.pfx -in mynick.pem
  1. Go to the Certificate Manager; "Preferences -> Advanced Options -> Encryption"
  2. Select "View Certificates" and from there you can import the mynick.pfx you just generated.
  3. Now connect to DareNET using SSL. ChatZilla will ask if you want to use the certificate for authentication. Say yes.

Note: you may have to confirm sending the certificate each time you connect, don't be alarmed! irc.darenet.org is a rotation, and it may ask you again if you get a different server from the one you previously connected to.

Once done, read how to add your certificate's fingerprint to N.