Sonicwall newbie question...

I have a Sonicwall 2040 appliance... its configured with a WAN LAN and
DMZ (all done prior to my arrival with the company). The WAN is our
public IP addresses, such as E-Mail and Web Servers, LAN is all
internal addresses, and i'm not quite sure what the DMZ is.

What I'm wanting to do is enable traffic from my WAN (specifically 1 IP
address) to my LAN (again, specifically 1 IP address) for remote access
purposes. I have a service setup on my firewall for Terminal Services
(port 3389), and a rule setup to all traffic from WAN to LAN for that
service. When I access my local server from the LAN, RDP works fine.
When I try from my public server, it says the service is not running or
it cannot find it.

Any ideas as to what I am doing wrong? Or what configuration option I
am missing?

Any thoughts are greatly appreciated and welcome.