I have a couple of Mitel sets, can't confirm that they are the 5313, but they looked similar. I tried them out for a while, then packed them a way a couple of years ago.
As I recall...first off, the sets have to have a SIP firmware load (off hand I can't remember where I got one, but most likely a search on the web), I think this required the use of a tftp server. After that it was just a matter of logging into the set and manually programming it using the GUI.