This assumes that your stick is seen by Dom0 kernel as **/dev/sdb** and you're mounting its first partition, so **/dev/sdb1** (the usual case); "w" means read/write, if you not want that, use "r" for read only.
This will attach the device as "/dev/xvdi" in VM. You can also choose different name (eg. xvdj) to connect multiple devices, but the others (not xvdi) will require mounting from cmdline in VM...
Needless to say, we will be adding support for USB mounting to our GUI Qubes Manager so that the user will not need to use console in Dom0 -- just click a button. Coming soon, stay tuned!