DHCP server is a server that assigns IP addresses and correct routing information to each user on the network (assuming they are set to obtain information from a DHCP server).
Unless the devices on your network connect to the MT DHCP server they won't get the correct information to enable them to communicate with the internet.
For example if the MT routerboard/os is set to act as a DHCP server you will need to manually input routing information, including DNS servers into the MT itself. Additionally If you want the to manually assign network information on each network device then you'll need the MT to have a route for DNS information.
I.e. if you want to use DHCP server you should also put DNS information in under IP / DNS.
I.e. if you don't want to use DHCP server you ONLY need to put DNS information under IP / DNS.
It may be worth reading up on what a DHCP server does and what DNS is, as this may help you understand the problem.
http://wiki.mikrotik.com/wiki/DHCP_Server
http://wiki.mikrotik.com/wiki/DNS
Josh (who is full of festive cheer!)