You've got to have a serial cable hooked up to the serial port. The management interface is via the serial port, a console based interface. You can use Hypterterm (among others) to get in. The default password is 1234 (see page 2-5 of the manual located <A HREF="http://www.zyxel.com/product/dsl/p642s.htm" target="_new">Here</A>)
<i>It's always the one thing you never suspected.</i>