The newer Extreme Networks XOS has a nice auto-secure sort of feature when you first set them up that locks down several things, one of them being SNMP. I was trying to add some X440G2 switches to Cacti and was having trouble enabling SNMP. These are the commands that seemed to work.
For reference, this was on version 184.108.40.206-patch1-3 for ExtremeXOS (X440G2-48t-10G4) switches.
configure snmp add community readonly public
enable snmp access
enable snmp community public
enable snmp access vr "VR-Default"
configure snmpv3 add community "public" name "public" user "v1v2c_ro"
Note that you might not need the last line, its sort of bad practice to use the name public also!