If you want to add a Manage DNS link on the My Domains listing page. You can do the following modification
Open templates/youractivetemplate/clientareadomains.tpl
Locate the following code
<li><a href="clientarea.php?action=domaindetails&id={$domain.id}#tabNameservers"><i class="glyphicon glyphicon-globe"></i> {$LANG.domainmanagens}</a></li>
<li><a href="clientarea.php?action=domaincontacts&domainid={$domain.id}"><i class="glyphicon glyphicon-user"></i> {$LANG.domaincontactinfoedit}</a></li>
<li><a href="clientarea.php?action=domaindetails&id={$domain.id}#tabAutorenew"><i class="glyphicon glyphicon-globe"></i> {$LANG.domainautorenewstatus}</a></li>Now, insert the following to the list
<li><a href="index.php?m=dnssuite&action=manage&domainid={$domain.id}"><i class="glyphicon glyphicon-globe"></i> {$LANG.yourmanagednsvariable}</a></li>You will need to add the {$LANG.yourmanagednsvariable} to your language files or hardcode it in