To enable statistics collection for a device:
In your left panel of XMC select Network, then go to Devices tab. Right-click on your devices (before that you can do multiple selection with CTRL and/or SHIFT buttons), hover on "Device" and select "Collect Device Statistics". You can select Monitor or Historical mode, depending on your requirements.
To enable statistics collection for interfaces:
In Network->Devices right-click on your device, go to View->Interfaces. In new tab you should see interfaces and be able to right-click on one or more of them and enable statistics collection right away. Two modes are available as well.
More in the manual: https://emc.extremenetworks.com/content/oneview/docs/l_ov_get_start.htm?Highlight=enable%20statistics%20collection#enable_data_collection
Hope that helps,
Tomasz