I cannot see the DNS Management button/link on the domaindetails page
You need to activate the "DNS Management" feature for the domain tld in WHMCS.
Goto Setup -> Domain Pricing and tick the box "DNS Management" for each of the domain tld's you want to offer DNS Management.
If you have domains registered before you have activated this feature, then you need to activate this for each domain manually under the clients domain tab by ticking the box "Management Tools" -> DNS Management.
You can activate the DNS Management Addon in WHMCS for all registered domains using the FREE LC & LB Tools v2 » "Manage TLD's & Domains"