Turning on DNS Managment is sort of a two part process.
First, you need to select DNS Managment on the domain. Each domain can have it's own DNS Management settings, and they can either be turned on or off for each domain in your inventory.
Second, the DNS Servers for any domain in which you want DNS Management active will have to be:
In other words, you can't set a domain to BOTH have custom DNS servers (ns1.somehostingcompany.com) and to also want to manage the DNS locally. When the domain is set to ns1.somehostingcompany.com, then that basically points the domain to ''somehostingcompany.com's'' DNS servers, and then looks there for the rest of the DNS Zone info. If you want to manage (edit) the DNS Zone locally, then you need to point the domain to the name-services.com DNS server, and then you can edit the zone file locally.
It is also important to understand - whenever you change DNS servers, that affects the hosting. So, if you currently have a website online and working, then the DNS servers will be pointed to that server's DNS's, and changing the DNS pointers on the domain name will by definition point the DNS pointers somewhere else and the hosting will be affected.