Shopsprays Punchout plugin is installed as a nuget package.
In order to do that you need access to Shopsprays nuget feed. Send an email to firstname.lastname@example.org to get access.
You will be given an account where you can retrieve the pre-authenticated url to the feed. You can now install the Shopspray.Punchout.Litium package.
Continue the installation process with the configuration stage.