You can enable bandwidth monitoring in the management console under "Network Services -> NetFlow -> Calculate Bandwidth", please save the configuration after you make the change. If you don't see bandwidth data within 5-10 minutes then please restart the service under "Network Services".
Top talkers should already be displayed under the main NetFlow page, under Network -> NetFlow -> History. Every section (e.g. hostnames) will show the top items. You can select a host to see which other hosts it is communicating with. Is that what you are looking for?