unfortunalety is not all documented very well at the moment, so it is sometimes hard to find out things when you are new on arduino. i find this out also
the core team seems working on new things, so they can’t do all at the same time.
to your questions : it is possible to send datas over usb to the board. just take an environment like max, processing, director and so on or a more specific program for serial transmit/receive like zterm.
there is a site about the theme : http://www.tigoe.net/pcomp/serial-director.shtml
i also tried with the serial xtra example program and it works very fine. be sure that you select the right port (name) and the right configuration for it. this is necessary at both sides : in the arduino tools menu as well as on director (normaly 9600 baud 8 n 1)
tried it and come back if you have any additional questions