Hello roboscorpion,
It all depends on what name server are you using, if you are using your company's name server then you will need to edit the ZONE on the company's server but if you are using ours then you will can manage the DNS ZONE from the Plesk control panel.
Also
http://ip-address/~username will not work on Windows, try this one:
http://ip-address/$sitepreview/mydomain.co.uk/