Before you can list new nameservers on a domain, you must first register them with the Registry. This is done through the Manage interface at https://manage.opensrs.net/
Login to the domain you wish to create the nameservers under and then click the Nameservers/DNS link at the top. Scroll down on the page and look for " If you want to create or modify a nameserver which is based on [domain name] click here. " Click the link to open the nameserver creation page.
Once you input the information and click the Create Name Server button, the nameserver is created at the registry immediately and can be used.
*Important* After the nameserver is created, it will only work within the TLD it was created under. To use the nameserver on all TLDs, also add it using the "Add Nameserver to All Foreign Registries" link in your reseller interface.