the username and password is the same what you used during the installation. so if you run the installation as "USER1" and password is "Password1" then you can login as "USER1" and password "Password1". ExtremeManagementCenter (NetSight) is by default using authentication of the underlay operating system.
if you deploy Virtual machine, then you are asked by initial wizard (on the console) for root username and password = you can use that.
Regards
Zdeněk Pala