I'm using beagleboard rev. C3 and I have built an Angstrom Narcissus
distribution using the online builder. Now I am facing problems with
the USB - it seems that the USB is not turned ON! So I cannot use a
mouse nor any other external USB device. I suppose there is a problem
with loading the EHCI driver. How can I check this and enable the USB?

EHCI cannot support a mouse or low speed device because it is High Speed only. Make sure you are using an external HUB between the USB port and the mouse.