Hi
USB is a bus. You cannot write/read to/from the PCI bus through LV, and the same is with USB. There are no read/write VI's. It's not to compare with RS232, where you have read/write VI's in LV.
Check this link for further informations:
http://digital.ni.com/public.nsf/websearch/42B604435E3F9604862565E20043CD30?OpenDocument
Regards,
Luca