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