You'll have to configure the device you want to monitor first. In my case, i'm monitoring the internet facing port on my Cisco 1841 router. I THINK these are the only commands you need to enter on that particular type of device:
snmp-server community <putyourreadwritepasswordstringhere> RW
snmp-server community <putyourreadonlypasswordstringhere> RO
snmp-server enable traps tty
You would THEN add <putyourreadwritepasswordstringhere> or <putyourreadonlypasswordstringhere> as your community string in the Bandwidth Monitor.
It's been a while since i configured mine...believe this is all you need. It really works like a champ once up and running.