Are you using the plugin manager on the Projects tab or are you using the intelxdk.config.additions.xml file? You should not be using the additions file to add plugins.
Look at the error message that you are getting. It is complaining about a missing plugin.xml file.
Take a look at another plugin, https://github.com/phonegap/phonegap-plugin-barcodescanner, it DOES contain the plugin xml file. I am afraid this plugin will not be work with the XDK in its current form. I recommend finding a different plugin that does something similar to what you are trying to do.