This version of xorg does not require a configuration file. Without it, xorg itself probes for the best configuration which as you noticed, unfortunatly does not include support of mouse wheels.
In a nutshell, I would recommend to create a default config file using the command 'X -configure', and then copy it to /etc/X11/xorg.conf and edit to your liking. The default config file will not work BTW. The mouse needs to be configured from /dev/mouse to /dev/psaux.
Good luck, and please feel free to update our wiki at http://wiki.caosity.org