Error USRP

I am new in the GNU Radio community and OpenBTS and I am looking for
help to get started. if I do:
cd /usr/share/gnuradio/examples/python/usrp
I unfortunately get this:

Testing 2MB/sec… usrp: failed to find usrp[0]
Traceback (most recent call last):
File “./”, line 106, in
main ()
File “./”, line 96, in main
ok = run_test (rate, verbose)
File “./”, line 63, in run_test
usrp_tx = usrp.sink_s (0, tx_interp)
line 2742, in sink_s
return _usrp_swig.sink_s(*args, **kwargs)
RuntimeError: can’t open usrp

…and i’m sure that i’m in group usrp…please HELP ME!