Viewing an old SNB forum post, someone said to use the username, not the email address.
Thank you again! I played around with the settings and did get an e-mail which confirmed I had gotten it right. For reference the setup for DNS-O-Matic is:
Hostname: 'all.dnsomatic.com' (Don't ask me why, but it's what works)
User name or e-mail address : 'username' (Using e-mail didn't work)
Password: 'password'
I used DNS-O-Matic with my old D-Link as well and it's a great service where you can do all your DNS related operations through the router not having to run and scripts and/or clients on a machine in your network. They support all major dyndns services in addition to OpenDNS of course (I'm using them for for parental controls).