These phones are not SIP out of the box, they initially need to be placed in SIP mode (the default is MiNET mode, or Mitel Network mode). Then you web to the phone and enter a username of admin and a password of <phone model> which would be 5212 in this case. In the web interface you can setup all this information.
All the information on how to do this is at the link you listed above, although you kind of have to poke around in the Mitel sections to find it all.