RMC6 can use one of two network monitors - WinPcap or the Applian Network Monitor.
Either you don't have one of them installed or something on your computer is blocking their function. Check the Windows Control Panel to see if they are present. If either is present, make sure that is selected in RMC6's Settings. If neither are present, install one or the other from Settings.
Since you would have installed either during the RMC6 installation process, most likely it got uninstalled or is being blocked.