SMP support isn’t built in to m0n0wall, and the current versions have no add-on SMP support available. m0n0wall will run on SMP systems, however it will only utilize one processor.
Michael's SMP support hasn't been updated in quite some time, and will not work with current m0n0wall releases.
Michael Iedema has written a program to automatically add SMP support to a m0n0wall release, which is available from http://www.michael-i.com/files/projects/m0n0smp.
The script requires pseudo-device
vn built into your kernel. When first run, it downloads
the latest SMP kernel from Michael’s site and updates the image. The
--update flag will re-download the SMP kernel in the event that Michael
releases a new revision of the kernel. Michael also has a pre-built copy
of the latest generic-pc image with SMP available for download from his
page.