Which mouse is it? Serial, PS/2 or USB?
If it's serial in COM1, create a link (as root)
cd dev
ln -s ttyS0 mouse
For COM2, replace ttyS0 with ttyS1. For PS/2, replace ttyS0 with psaux. I'm not sure about USB.
Note: xf86config can put the right values into your X config files, too. I accidentally said earlier that the command is xf86setup, that's obsolete.