My WAN-interface does not automatically obtain an IP-address so I need to launch the command 'dhclient' to obtain an IP-address from my ISP.
SME server already launches dhclient, if you have selected DHCP in the configuration console.
If the interface is not obtaining an address, you need to determine why it isn't. Start with log files /var/log/wan/current and /var/log/messages.
Open a bug report in the bug tracker if you cannot identify the problem.