Problem with moving domain name to X10

[dart18pt]

I have been reading several posts on this topic, and I cannot solve my problem.
O have a domain name www.dart18.com.pt that I want to point to my account here at X10hosting.
I have changed the name servers at the source, and tried adding the domain as a parked and also as a new domain.
In both cases I get an error

Unable to add the domain name − Could not determine the nameserver IP addresses for “dart18.com.pt”. Please make sure that the domain is registered with a valid domain registrar.​

When checking on whois this is what you get. They say the name servers cannot change because my domain at X10 does not exist.
I surely need HELP...

Nome de domínio / Domain Name: dart18.com.pt
Data de registo / Creation Date (dd/mm/yyyy): 06/09/2006
Data de expiração / Expiration Date (dd/mm/yyyy): 02/11/2015
Estado / Status: ACTIVE

Titular / Registrant
AlmourolTec - Servicos de Informatica e Internet Lda
Estrada Nacional 3 - 9-C
Constancia
2250-028 Constancia
Email:

@buydomain.pt

Entidade Gestora / Billing Contact
AlmourolTec - Servicos de Informatica e Internet Lda
Email:

@buydomain.pt

Responsável Técnico / Tech Contact
AlmourolTec - Servicos de Informatica e Internet Lda
Email:

@buydomain.pt

Nameserver Information
Nameserver: dart18.com.pt NS ns2.parking.webserver.pt.
Nameserver: dart18.com.pt NS ns1.parking.webserver.pt.