I don't think you have to mess with that one.
You should have THREE IP addresses assigned to this server. By default, they are not bound to the server. You would do this through the server admin control panel, not Plesk - they are different. (You may need to call G.D. support on this one to walk you through.)
Second, mark this well: there is a reason you have three. You will need to assign two of those to a primary and secondary DNS so that your DNS report for your domain(s) come up clean. The mistake often made is to a) bind only one IP to your name server and assign NS1 and NS2 to that IP, or 2) dedicate two IP's away to SSL certs, which means you have to get more IP's. Your NS1 and NS2 need to be different IP's*, and need dedicated IP's for any domains you use as a name server.
Go to the domains you want to use for name servers, dedicate the IP's to those domains. Go into DNS setup for those domains, add A records for the name servers: ns1.domain.com, ns2.domain.com, etc. You may be able to do this as a subdomain of the domain you are using if you only have one domain on the server. (Get support on the phone, this info may be incomplete!)
While you're in there, add an SPF record, this will help avert future problems with email. Click Add Record, select TXT as the record type, add
v=spf1 a mx include:[ROOT SERVER].net ~all
Where "[ROOT SERVER]" is the name of GD's outgoing MAIL SERVER. In my case that's just "secureserver.net." Yours may or may not be the same. Understand this is not your mail server, mail.yourdomain.com, this is G.D.'s mail relay server. All your mail will have to pass through this (it's how they control relay limits.)
Once your DNS servers are set up, go into your domain name registration account (hopefully with G.D.) Go to the domain manager. Get on the page to edit the domain name info for the domain you want to use as the name server. Lower left, look for "Host Summary", it should be blank. Click add.
Now add the two name servers you created. Don't add the full NS name (ns1.domain.com) just assign two NS's, NS1 and NS2.
* The above scenario is not perfect - name servers for a given domain SHOULD be geographically diverse, in which case you might want to pay for an external DNS service, which adds up. But if you assign two unique IP's to your DNS servers, it is passable on the DNS report.
Again, been a while since I did this, so get support on the phone before making any changes - they are actually very helpful and unless you add a lot of domains or SSL certs to the dedi, it should only have to be done once.
After it's all set up, before loading any files go to DNS reports and run a report. Address any red flags one by one by tweaking the DNS/Mail server settings. As you do any of this, keep in mind it takes time to propagate, just like moving a domain, don't be alarmed if the changes don't update immediately.