It's tricky, if memory serves (I did this in 2007) on that exact hardware..I found a guide that I had to modify...oh yeah, and I had to build a keyboard connector for it.
I had to pop open the box, and use one (probably the only one) PCI 32-bit slot in there to hold a dirt-simple vga card, and somewhere on that board you'll find a 5 or 6 pin keyboard pinout which is very standard, and you'll see a mini-IDE connector. The logic board should contain 8MB of flash if I'm remembering correctly.
i THINK what I ended up doing (in a moment of madness) was I got the latest m0n0wall image on an IDE drive, hooked that up, and used the command line to dd the image onto the on-board flash. It was quite a journey for me at the time. I seem to recall most of the documentation was misleading.
It did eventually work though, but 8MB of flash is no longer enough to support the latest m0n0wall releases.
Good luck, lemme know if you need more help, I might be usefull....maybe...kinda
volumetricsteve@gmail.com-or-
@VolumetricSteve