If your getting the error CONNECT ERROR: Couldn’t resolve host ‘magento-core’ while attempting to install or upgrade magento or a magento extension its becasue of some changes depending on the version of magento you are running.
Alot of tutorials will reference extension installs something like: magento-core/Mage_All_Latest
With the newer versions of magento connect that changed you now use the format: http://connect20.magentocommerce.com/community/Mage_All_Latest
Basically you replace “magento-core/” with http://connect20.magentocommerce.com/community/
Hope this helps!