If you have other 'device' stats then munin-node at the 'device' platform observed is working.
So you can start by check munin-node list of plugins reported by the device by 'telneting' to port 4949, as explained at munin-node test wiki http://munin-monitoring.org/wiki/MuninTroubleshooting .
Then I guess you have to check what interfaces you want to monitor and if munin-plugin at step 6 of your question link forum.
In other words, the line
"ln -s /usr/local/share/munin/plugins/if_ if_vr0"
only works if the script/aplication at "/usr/local/share/munin/plugins/if_" knows how to handle interfaces of type vr0.
Assuming all is compatible you just need to know the interface name like "eth0" or "e0001" check ifconfig or other command that shows interface naming, then adapt or add those interfaces to munin node:
device$ ln -s /usr/local/share/munin/plugins/if_ if_eth0
device$ ln -s /usr/local/share/munin/plugins/if_ if_e0001
Then you could test also to see if plugin is generating the rigth data:
device$ cd /usr/local/etc/munin/plugins/
device$ ./if_eth0 config
device$ ./if_eth0
device$ ./if_e0001 config
device$ ./if_e0001
Last, from last comment on your question link forum someone say that "need to end your scripts in *.sh" so perhaps you should try also:
device$ ln -s /usr/local/share/munin/plugins/if_ if_eth0.sh
device$ ln -s /usr/local/share/munin/plugins/if_ if_e0001.sh
device$ cd /usr/local/etc/munin/plugins/
device$ ./if_eth0.sh config
(list of this munin graph axis, labels and title defs)
device$ ./if_eth0.sh
(list of this munin graph values)
device$ ./if_e0001.sh config
(list of this munin graph axis, labels and title defs)
device$ ./if_e0001.sh
(list of this munin graph values)
Next you will write your own plugins, or at least if you need to change code from 'if_' script you want to check the how to at http://munin-monitoring.org/wiki/HowToWritePlugins .