Verbose means debug. Sorry for the confusion.
The file is /usr/local/Extreme_Networks/NetSight/appdata/logs/server.log in the filesystem (based on your installation path. You can see the content of the log also through the OneView gui.
Where can I enable debug? I don't see it in Administration -> Diagnostics. With Level:Diagnostic I see a drop- down menu where I can choose the level (information, critical, verbose...). Something like that?