Can't find USB-Serial device

Hello, I'm working on a Beagleboard, with Angström (custom image,
kernel 3.something)

My application needs to use several peripherals over RS232, so I'm
using a 4-port USB-serial hub, and also a regular Prolific USB-serial.
Both are detected correctly and show up on dmesg, but the usual /dev/
ttyUSB0 or /dev/ttyACM0 doesn't get created. Does anyone know where
the device gets created, or what I can do to create it?

Besides, I can use the /dev/ttyO2 RS232 port, but no idea where the
ttyO{0,1,3} are. Are they available in the board and easily solderable?

Just a couple of thoughts:

Sometimes the devices are “hidden”, make sure you use ls -a.
When getting my serial ports to work I had to make sure that the device permissions were at 777.
If the serial ports are not exposed on the expansion port you’re out of luck. Check the HW reference manual, they may be multiplexed.