Patience! I have used a DWL-G520. Just add it as an option on the m0n0wall (not the gui, but the boot screen). Then go into the GUI and your options will be there.
Where you can change options from boot?
?
In the log file - kernel: pci0: <network> at device 10.0 (no driver attached)
Version m0nowall - 1.3b4 (upgrage from 1.231)
Pc generic- PIII 600 - 512Mb ram