The AP600 replaced the AP450. I had the same problem on a 450 ones. You have to connect a terminal(emulator) to the serial port with the console cable. recycle the AP and during boot you can break into the boot prompt, with I believe a Control-C. But it'll show you the message. once in a boot> prompt you can reset to defaults. which resets the username to "admin" with no password.
I don't remember the actual command. but a "?" will probably give you the available boot> commands.
regards,
peter
pfischer@atmit.com