Active checks would make no sense here, since most of the time the temperature should be OK.
But you want to get notified in case of cooling problems and over heating.
First you need a plugin for munin:
Verifiy that this plugin is working prior doing anything else!
Also adopt the warn and crit values to your needs.
Now you need nsca installed on your munin master node.
step is a proper munin master configuration for your system that has
the USB thermometer connected. We assume that the system has the name
intranet and that you have configured munin to make use of some
domains. intranet is located in domain intern.
contact.nagios.command /usr/sbin/send_nsca -H <nagios server IP> -c /etc/send_nsca.cfg
address <your systems IP>
If you omit the "notify_alias" part all alarms will be sent from the given system name plus domain appended (intranet.intern).
With the notify alias you can make sure that nagios receives the alarm for the proper system.
Now the nagios system needs to get configured.
Make sure you have nsca installed and running.
Now enable passive chacks in nagios.conf and create a service for the temperature alarm: