There are several knowledge base articles and screenshots for monitoring network devices. Here is a screencast for monitoring a network device with SNMP & EventSentry:
Here is a knowledge base article with an example of how to enable SNMP on the Linux device:
By default, the "Disk Space" monitoring package is assigned globally so you won't need to assign that to the Linux device manually (it will already be done). I hope this helps and if you have any other questions, please let us know.