Another trick to get management ip if you do not know it, is to plug a cable into the management port. Then you can run wireshark on your machine, and see if you can see the ip address in any packets from the controller. You could to the same trick on the data ports. If enable management through data ports was selected, then you should be able to plug directly into one of those as well, and then ssh to that ip.
You can see the mgmt. port and serial port locations in the C25 Quick Install Guide located here:
https://extranet.extremenetworks.com/downloads/Pages/WirelessControllers.aspx