It's asking for the SNMP community string.
You need to add commands to your router to export the Netflow information to the collector. We use Cisco and I had to add the following to mine...
ip flow-export source Loopback0
ip flow-export version 5
ip flow-export destination 172.16.4.2 2058
no ip address
encapsulation frame-relay IETF
ip route-cache flow
When I added these the router the interfaces reporting show up in the Monitor. The IP address and port have to match the Ip address of the system running the monitor and the port you have selected. I do not remember if I ever used the add interface method but that method does require the SNMP string so that the monitor can pull the resource list from your device.