Marksystem use a number of different sets of nameservers – which set of nameservers you use for your domain depends on your setup. Generally, if you have a hosting account with Marksystem, you should refer to your ‘Hosting Account Welcome Email’ for the correct set of nameservers to use. If you do not have a hosting account with Marksystem, you should use the nameservers ns1.marksystem.company and ns2.marksystem.company
If you are unsure after the above, please submit a ticket and a member of the team will be happy to assist further.