At the moment we use a plus sign between vendor name and package name for M1 extensions in package.xml. This is ok for installing via link, but installation of package using downloaded file will create errors. Installation of package from file uses different validation that does not allow to use '+' sign in package name.
It is recommended to install extensions for M1.X.X through the Magento Connect Manager.
The solution?
Install the package via link:
http://info2.magento.com/rs/magentosoftware/images/InstallingMagentoConnectExtensions4%200.pdf
OR
Add unique Connect channel URL (see image below)
