What specific error are you getting? Do you mind posting a screenshot?
The Network Device Monitor is for monitoring a specific SNMP values (one OID). If you are looking to monitor bandwidth, you will need to enter the appropriate OID for Transmit or Receive. Assuming you have already enabled SNMP on the device and added it to NDM, click the Setting icon (gear button in upper right) and then click Browse MIBs and select the appropriate counter from the IF-MIB (184.108.40.206.220.127.116.11.1)(ex: ifOutOctets).