I have setup microtik router configuration. I do not know much about how to assign a static ip to NAS or any other devices.
What I want to do is define a static ip for NAS so that it will automatically take that ip. What I have done so far is go to "IP"-->"ARP"-->"ADD NEW" added mac address and ip. Then go to "DHCP SERVER"-->"leases"-->"ADD NEW", added mac address and ip,
and it is not working. Any help?

In winbox go to DHCP Server -> Leases, find already assigned lease for your device, righ-click on it and in menu click Make Static. If IP address should be different from automatically assigned, open this lease with doubleclick and edit IP address. It must be in DHCP address range. Then restart NAS and it will take this address.

the static DHCP IP address that you specify does not have to be within the DHCP pool. i have 48 static DHCP leases on my home network, and my devices all work fine without them being in the DHCP pool.