Hi,
If you are using the option Enable DMZ from Network Menu. Overthere you have 2 options - to put a subnet or a range of IPs. But the 2 options require to put a public IP or IPs. Than on the DMZ port you plug the machine which will receive this trafic and you configure this machine with this same public IP.
This router do not support NAT on the DMZ zone - what I mean is that you cannot assign VLAN with DHCP to the DMZ zone. This feature is support only by SA500 and ISA500 from the small business range.