This addon is official but is not included within Construct 2 itself and must be downloaded from this page.
Access the Steamworks API from the NW.js exporter.
WTF is this crap. No install instructions and all it gives you is some web file. This is not a C2 Plugin.
There are instructions for installing addons in the manual: scirra.com/manual/158/third-party-addons
There are further instructions here in the 'Documentation' tab: construct.net/construct-2/addons/83/greenworks/documentation