You need to have a community string setup on any devices you wish to poll. You should be able to get information on how to setup or find out the community string using your devices' documentation. Then run the Real-Time Bandwidth Monitor and enter the IP address of the first device you wish to monitor as well as its community string. The credential test makes sure that you have connectivity to the device and that the credential (community string) is correct. If your credential tests fails you are either using the wrong community string or something (firewall?) is blocking your connectivity from your host device to your target device.