kenwrites
For testing purposes, you can create any domain name you want in the admin console/Configure this server option (in a valid format eg mytestdomain.com), as the main domain name for your sme server.
Then set that domain name to resolve locally in server manager Domains panel, so that it does not need to be a real domain as far as external Internet users are concerned. You just want it to be resolvable locally on your LAN for test purposes. If sme is not your LAN (proxy) gateway, then point your browser proxy at the sme server IP.
I believe dyndns now charges for their services, so I'll try to set up dynamic DNS through dnsdynamic.com
Pretty sure dyndns still allow 1 domain name for free per unique registered user, but you need to supply a credit card at registration time (it will not be charged unless you add more than 1 domain).
So you can still avail of a free dynamic domain at dyndyns.