If the domain is currently registered through Mr.Host, then you can make the DNS server changes via the Domain section of the Customer Control Panel.
If the domain is registered through another registrar, then this procedure would vary depending on their system.
Generally, they would have some sort of web-based interface, or online form you could use to make changes to your domain. This generally includes a username/password for your domain, that they would provide.
The DNS servers should be set to:
Primary: ns1.mrdns.com
Secondary: ns2.mrdns.com
When making the DNS server changes, make sure to remove all other DNS server, so ONLY the Mr.Host DNS servers are listed.