After updating to Joomla! 3.4.0, you may have encountered some extensions that did install in previous Joomla! versions are now throwing the message:
Archive does not exist
Unable to find install package
What is causing the error
Joomla! 3.4.0 introduced a new security feature, scanning uploaded files for potential dangerous code. For example, it scans uploaded archives for PHP code - something that should not be done in the Extensions Installer (com_installer). Fortunately, Joomla!
contributors already debugged and fixed this (pull request here) so if you don't want to wait until 3.4.1 is released, read this blog post.