K to add on a domain you must first register it with a domain register which usually cost 8.99 to renew it every year and something for a base cost for the first month.
Now if you have already bought the domain name, you must point the name servers to ns1.x10hosting.com and ns2.x10hosting.com and then do the add on or parked domain.
If your getting trouble from it after setting the name servers you could probably ask an admin with a post in this topic if they could try to set the parked/add on domain.