authorize zone transfers

[R Joseph Wright]

This is a pretty simple question, but I can't find the answer anywhere.
I've set up my own dns server to use as a primary and I want to use the
public dns service at granitecanyon.com for my secondary.  Their
instructions are very simple, but step one is something I can't find the
answer to:
Authorize ns1 and ns2.granitecanyon.com to perform zone xfers from your
primary nameserver.  
How do I do that?  Do I need to add a line to named.conf or what?