Is the domain still in "processing" status? (for example if you recently purchased it through Dynadot's expired auction). In that case you would need to wait a couple of days before the domain will be added to your account and after that is done your name servers can be changed.
If the domain isn't in "processing" status then you can change the name servers easily by selecting the domain first and then clicking on the "Nameservers" icon.
The .tel domain name cannot be used to display a business or personal website. The .tel web pages are uniform in display and the .tel domain owner has control over the information that is shown.
Unlike our other domain names where you can control such things as email forwarding, name servers etc. .tel domain names are administered through an interface provided by Telnic who also provide the hosting of the domain.
When a .tel domain name is registered, Telnic will send you an email with your Telnic login information. All content changes and updates for your .tel domain must be done through the login provided by Telnic.