Hi Jins,
You did not mention if you are using some DHCP server. In case you are using and if it is ver 6.3, I have something to bring to your notice. In ver 6.2 or less , dhcpd enable command would only accept the inside interface as an argument. But in 6.3, PIX Firewall versions 6.3 and later do not have this restriction. However, Firewall MC only allows you to enable the DHCP server on the inside interface. Importing the dhcp enable command on another interface causes an error.