DKIM setup with my own domain

[#1]

All my emails are sent by plus net servers via relay.plus.net.
I send my emails with my own domain name (@perdrix.co.uk) and inbound mails go via the Zen MX mail servers and are enveloped for my plus.com email addresses.
So far so good.
I now need to send mails to some Facebook email addresses (some friends only have these), but FB now requires mail sent to facebook.com email addresses to be DKIM signed.
How can I set this up - can I just add the plusnet public key information to my DKIM TXT records?  If so what is the correct incantation?
If not is there anything else I can do (e.g. send mail using my plus.com email addresses)?
Thanks
Dave

[#2]

Hi Dave,
Whether you send from your domain or your .plus.com address, DKIM signing is not supported by our mail platform.
It really shouldn't be required though, as evidenced by my post in your other thread here.