You cannot create a CPanel Add-on Domain until the DNS for the domain you are trying to add has propagated.
DNS Propagation usually takes about 24 hours after the DNS Nameservers are set.
The DNS Nameservers to use would be the same as for your main CPanel domain.
If we registered the domain for you and need to change the DNS Nameservers please open a support ticket.
In CPanel go to Domains -> Addon Domains
When an Addon Domain is created a new folder will be created for it.
By default this folder is created in the Home directory, but you can customize this after creation by editing the "Document Root" parameter and manually moving the folder.