By default, when you've run import on a distro, the cobbler server is configured as the default source in the kickstart assuming you have the $yum_repo_stanza and $yum_config_stanza variables in your kickstart (refer to the sample_end.ks we package to see where they go). Typically you will not setup a repo for the base distro, but you will usually setup repos for updates/security/EPEL etc.
If you added the distro manual with "distro add", you'll need to add the tree variable to the distros ksmeta:
$ cobbler distro edit --name=<your distro name> --in-place --ksmeta="tree=http://@@http_server@@/cblr/links/<dir>"
Where <dir> is the directory (symlink) in /var/www/cobbler/links/ that points at your distro.