Hi Sergi,
Have you taken a look at the following example:
http://zone.ni.com/devzone/cda/epd/p/id/4516
Also you may find section 3.2 USB RAW class to be helpful from this link:
USB Instrument Control Tutorial
http://zone.ni.com/devzone/cda/tut/p/id/4478
Also, see the following article for information on connecting to your device using a VISA Test Panel.
Connect Instruments via USB
http://www.ni.com/gettingstarted/setuphardware/instrumentcontrol/usbconnect.htm
Hope this helps!
Kaitlin N.
National Instruments
Applications Engineer