There are two aspects to a domain name 1) the registrar and 2) the DNS server it resides on. It is pretty common for the registering organisation to also provide DNS services. If they don't then they may well provide a redirection service. The registrar must provide a zone edit facility of some kind to manage your domain name, even if it's only the ability to set the DNS server.
Typically a zone edit facility enables the domain name owner to set the "A", "CName", "MX", etc. records in the DNS entry. If you PM me the name of the domain I will do some investigation for you.