grendl that documentation is very old now, and I wouldn't hold you breath waiting on an update. I would suggest you visit the m0n0wall repository:
http://svn.m0n0.ch/wall/branches/The newest source code branch is freebsd8 (based on freebsd 8.3). There's scripts that the developers made that will assist you in building m0n0wall from scratch. The following link below is a direct link to the scripts that are used when building m0n0wall:
http://svn.m0n0.ch/wall/branches/freebsd8/build/scripts/Read the readme file, and it will give you instructions on what you need to do.
go.sh - scripts is issued first which sets up the directories, downloads file etc
doall.sh - will execute the following scripts in order:
1makebuildenv.sh
2makebinaries.sh
3patchtools.sh
4buildkernel.sh
5makeimage.sh
I would not recommend using doall.sh. and instead execute each one of the 1-5 scripts individually. You will be able to see which script may fail for some reason. If you have any problems, post a response here and I will help you out. I'm almost a master at building m0n0wall using those scripts now since I've been using them almost everyday for awhile now.