Are you using the build in DHCP server that is part of SME?
IF so, when you installed the SME server and ran the Admin setup console, you would have the option at this point to set up your DHCP setting including the start /end of you DHCP scope and DNS Server.
If you have a keyboard and monitor connected, log in as ADMIN with your admin password (same as the root PW). If you are connected via you network, use Putty or telnet if you have enabled this access. This will launch the Server Console. In the Server Console select the "2. Configure this server" option. Just keep going next until you get to the screen that says "Select DHCP server configuration". Here you want to select the "ON" option. You will then be able to set your start IP address, End IP address and the IP Address of your current DNS Server. Don't forget to restart your server.
HTH
Darin