The yum command won't install from the local DVD it will download the package off the Internet from RedHat's servers.
if you wish to install a local DVD package you need to find the rpm file which if i remember correctly is in the RedHat directory on the DVD
then use the RPM command to install the package i.e.
rpm -ivh iscsi-initiator-utils*el5.x86_64.rpm
This will then install from the disk.
It appears that the package your trying to install using yum uses a security key that is not in your local key list.
This URL
http://www.art122-5.net/index.php/In...gnature:_NOKEY
Describes a similar problem and a suggested resolution (Which I have not tested), this will however download off the net not install from the DVD.