You might first verify the hardware can be seen by the software.
Code:
$ dmesg | grep -i mouse
$ lsusb | grep -i mouse
If either command returns blank, you'll know you have a hardware problem (though the second may be blank if you have a non-USB mouse).
I've never seen a case where X didn't automatically see the mouse, so I'm not sure how to diagnose further.
----- Edit
You should also be able to see a mouse device.
Code:
$ ls /dev/input/mou*