is this really the correct error message ?
/dev//dev/ttyS0 does not exist at all.
strange that you get an additional /dev in front.
otherwise if you use the USB board your serial device most likely is called /dev/ttyUSB0 (you can look this up with the dmesg command, after attaching it)
as already pointet out before by mellis, the RTXTX lib does not recognize USB serial devices, so you need to make a symlink
to a non existing /dev/ttySx, zb:
log in as root and then:
ln -s /dev/ttyUSB0 /dev/ttyS10
restart arduino and the new serialport should show up.
maek shure you have the rights to read and write from/to it.
best
erich