you need to set up SNMP on the Cisco router... here's an example:
snmp-server community SOMESPECIALSECRETPASSWORD RO 80
access-list 80 permit 10.10.10.10
Having the snmp-server reference access-list 80 will permit only those hosts permitted in access-list 80 to perform SNMP queries. Access-list 80 should list the IP address of your Dude server (and any other devices that will be polling it)
Once SNMP is set up on the Cisco, you'll need to set it up in The Dude.
Settings->SNMP->Add a new profile with SOMESPECIALSECRETPASSWORD as the community.
Now go to the Cisco's device settings in The Dude, select the correct SNMP profile. Within a few minutes, you should be able to go to the SNMP section of the device settings and see your interfaces and such...
At this point, you can use SNMP to monitor bandwidth.
Double-click a link coming off a Cisco. Set the device to your Cisco's name, set Mastering Type to SNMP, then select the appropriate interface!