Where to download things
There are three things you need to make sure you have downloaded and version matched – NW.js, Greenworks and the Steamworks SDK. For example, if you want to use Greenworks v0.52.0, then you'll need Steam API v1.50 and NW.js v0.52.0.
NW.js
Construct automatically downloads NW.js when you export. Each NW.js version is based off an equivalent Chrome release, so the corresponding versions are also listed (under the name Chromium, which is the browser engine that powers both Chrome and NW.js). By default, Construct will always use the latest version available.
You can also download NW.js versions ahead of time and manage your current versions in the NW.js Version Manager:
You don't have to download NW.js in advance as it will download when you export, but it can make the export process longer, particularly if you have a slow internet connection.
Greenworks
Greenworks is an official plugin, but it's not built into Construct 3 – you'll need to download it from the Addon Exchange.
As previously mentioned, each release of the Greenworks plugin must be used with a specific version of NW.js, and a specific version of the Steamworks SDK. The release notes for each release of the Greenworks plugin specify which versions you must use it with.
If Steam is unavailable or something has gone wrong the condition Is available will be false, so check it's true before using Steam features. There should be a message in the dev tools console if something went wrong (use the 'Show dev tools' action in the NW.js plugin to check).
Unfortunately, Greenworks does not currently work in preview mode.
Greenworks needs to be installed via the Addon Manager and if you need help with that, then please check the manual guide covering Third Party Addons.
Steamworks SDK
The Steamworks SDK is available to download from the Steamworks Dashboard, though bear in mind you'll need to have registered an account with Steamworks to be able to access this. There are loads of versions of the SDK to download, so make sure you have the one that matches your Greenworks version.
If you want more information about the Steamworks SDK, be sure to read their documentation.