You can find that on your registrar's panel (on the same place where you brought it, you'll find a way to "manage" your domains)
On the management, you need to search for "Update/Change DNS" o "Name Servers" and then proceed to edit them.
Depending the registrar, the process will vary, but they will give you a option to set your own name servers, in such case use the next two (at least two nameservers are required)
NS1.X10HOSTING.COM
NS2.X10HOSTING.COM
Then, just accept and await them to get updated worldwide (this can take up to 24 hours)
Also remember to add the domains you have brought to your parked/add-on domain list here in X10 hosting using the cpanel.