Download and install (or re-install) the latest Combo Update for Mac OS X Server 10.6, available from Apple Support Downloads. This will restore the software necessary to configure Lights-Out Management.
Xserve: Cannot configure Lights-Out Management after installing Remote Desktop
After installing the Admin package of Apple Remote Desktop 3.2 or later on an Intel-based Xserve running Mac OS X Server v10.6 or later, you may not be able to configure Lights-Out Management. "Configure Local Machine" is disabled in Server Monitor, and the /usr/bin/ipmitool command is missing.