Are you following the installation instructions here?:
http://m0n0.ch/wall/installation_embedded.phpI don't have the hardware to run embedded.
On x86 PC, you have to have a monitor and keyboard attached to do the minimal initial configuration. You have to tell it which ethernet interface will be LAN, which one will be WAN, and which if any will be OPT. There are no default settings for this. Once initially configured, you can detach the monitor and keyboard and continue with the rest of configuration thru the GUI via another LAN PC and web browser.
On embedded, I am fairly certain you need to do initial configuration thru the serial console port. This is also how you can reset the LAN IP address if you forgot it, or reset it to the as freshly installed defaults.
Try talking to it via the console.