Hi Pat. You are correct in your assumption that the Pluto Core is meant to be used as your home router. In this case your External NIC is connected to the Internet, and your Internal NIC acts as a DHCP server, DNS server and gateway for your internal network.
In your situation (i.e. only one NIC), as I understand it, you will need to assign a static IP to that NIC. Pluto will set up a DHCP server on this NIC as well, so make sure you disable your router's DHCP server or any other DHCP server you have on your network. This is so that Pluto can hand out PXE boot images to Media Directors. This also explains why Pluto isn't letting you use DHCP to obtain an IP for your only NIC - something would break if your DHCP server (Pluto) needed to obtain a DHCP address from somewhere else. Not to mention you'd then have two DHCP servers on your network.
If you put a second NIC in, you will be able to use DHCP on your External NIC, but your Internal (because it's acting as a gateway and DHCP and DNS servers) will have to be static.
Hope that clears things up.