You will need the remote computer's IP address or DNS name to connect to it. You can obtain it in the following ways:
If the Radmin Server installed on the remote computer, move the mouse cursor over tray icon and IP address will appear.
Run the ipconfig.exe command from the remote computer's command-line and examine the 'IP address' field.
To obtain DNS name, run the ipconfig.exe with /all command-line switch and examine the 'host name' and 'primary dns suffix' fields.