You have to add the domain as addon domain through your cpanel, and always use the same nameservers that you got on the welcome email, the nameserver change have to be done at your domain registrar which seems to be in this case, namecheap.
The easy way would be DNS delegation to your cpanel host.
Login to NameCheap's control panel, manage domain and choose "Trasfer DNS to Webhost". Then in fields marked 1 and 2 insert your host name server names. Hope this helps!