You’re trying to do a static DHCP lease (LAN IP assignment) for a particualr user? You can do this one of two ways, either by MAC address, or by User Id. For User ID your DHCP client has to include that option (DHCP option 61) in it’s request. For either you must add a static lease definition which includes one of those fields to identify the request. http://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Server#Leases
Also see this recent thread for examples: http://forum.mikrotik.com/t/dhcp-clientid/53010/1