Not familiar with Netopia routers, but if you have your pinhole configured to forward TCP port 3389 to the private IP address of the machine your are trying to connect to you should be good.
Try 'telnet xxx.xxx.xxx.xxx 3389' from outside your network, replacing the xxx.xxx.xxx.xxx with the public IP address of your router. You should receive a blank screen with a flashing cursor if all is well. If you receive an error, it may give you a better idea what is going on.