I'll start with the two three obvious questions:
Edit: 3. Have you tried it with a different USB cable?
Good luck.
And check that the usb cable, at the tablet end, is in the correct orientation. I have a USB microphone (a Blue Snowball), that will light up as if it's connected, but won't work. I found that the cable can be inserted upside down (don't get me started on the stupidness of the "square" end of usb cables...).