use the hostnames and addresses option in the server manager.
i have been to that panel 457654367346 times and never bothered to read:
"The ethernet address is optional and causes the DHCP server to statically bind the local IP address to the computer with this ethernet address. If specified, it must be of the form "AA:BB:CC:DD:EE:FF" and must contain only thenumbers 0-9 and the letters A-F."
thank you
zoran