The Invalid package identifier provided error occurs when there is a problem with the installation of an extension through Magento Connect Manager.

Often it is caused by the fact that the chosen extension is provided as a beta release.

The switch between the stable and the beta states can be easily completed through the Magento Connect Manager->Settings->Preferred State.

Alternatively, you can install an extension using the following shell command:

./pear install Magento_extension_key

In the above command line replace Magento_extension_key with the exact key for the chosen extension.

In the following example it is shown how to install a classic theme extension through the command line:

./pear install magento-community/magento_classic_theme_free
downloading magento_classic_theme_free-1.4.1.2.tgz ...
Starting to download magento_classic_theme_free-1.4.1.2.tgz (3,687,973 bytes)
.......................done: 3,687,973 bytes
install ok: channel://connect.magentocommerce.com/community/magento_classic_theme_free-1.4.1.2