Running services provided by No-IP, DNS2go, etc., will allow you to map your dynamic IP to a hostname freely provided by these services. For example, you can choose to reserve 'tony.no-ip.org' which will then map to the current IP address your ISP has provided you with.
I personally have just started using this service and have found it's very stable and reliable. The best part about it is that it's free of charge, plus you can customize the rate upon which your PC or server located on the local network will update the central servers of the service provider to reflect your dynamic IP address.
Once this is complete, all you need is to setup a simple port forwarding rule on your ADSL router and access services on your local network. In Radmin's case, it's port 4899 (default port). Assuming you haven't changed the port which the service listens on, this is what you'll need to port forward.
Coming to your router's configuration, because I am not aware of the interface, I'll describe what you'll need to do so you can then find your way through the menus and complete it.
Every router these days supports NAT/NAPT, which is essentially forwarding requests on the public interface of your router to the inside network it is protecting. Requirements here is to select the router's external interface (usually one), the port number that will be forwarded, the internal IP address to which you wish to forward all incoming requests and finally the internal port number.
In fact, there is quite a good logical break down on the rules we need to create to accomplish the above, and it goes something like this:
'If someone hits my WAN interface on port 4899, then forward them to the internal machine with IP address 192.168.0.10 and port number 4899, where 192.168.0.10 is the internal server you're trying to connect to and port 4899 the Radmin service running on the server.'