I'm beginner in Mikrotik and I need a help how to setup DHCP server to lease two different DNS (normal Google and Unotelly-USA-DNS) depending on devices (by MAC address).
Please help me to setup (better in WinBox).
I tried to find this solution in forum but found only one post http://forum.mikrotik.com/viewtopic.php?f=2&t=88803, and I see that there problem is still open.

I have some devices which can obtain IP and DNS only from DHCP (Roku). I can't setup manually for this devices IP and DNS.
In the same network I hava also other devices and for this I have to use 'normal' DNS.
This is a reason whe I need to setup in that way my DHCP server.
Anyone can help me ?

First use the Leases tab in DHCP Server to set the "special" device to a static IP.

Then copy your existing networks entry in the DHCP Server using Winbox. In the copy change the address to A.B.C.D/32 where AB>B>C>D is the static IP address for the "special" device. Now edit the DNS settings for the "copied" Network settings and save it. You should now have two Network settings - one for the subnet and one for the individual IP of the special device.

Thank's for help.

Probably I have done something wrong because after your procedure (after COPY and OK) I got the message " Couldn't add New DHCP Lease - already have static lease for this clieny (6)"

I don't know what to do next, and why this happened ?

Look in the leases - sounds like you already have a static lease for that MAC address.