Use Autodesk Account or the Autodesk App Store. Steps:

: A classic suite of tools used to automate the generation of roof trusses, gable walls, and other complex structural framing. Site Designer

The Revit Extensions for 2018 are a collection of specialized tools that extend Revit’s functionality in the following areas:

Autodesk provides specific add-ins and extensions (such as Site Designer or Structural Precast) through your subscription management page.

Autodesk no longer officially supports Revit 2018 (support ended in 2019–2020). Direct download links from Autodesk’s main website have been removed. However, legacy users with active subscription contracts may still access these files via the Autodesk Account portal.

: Download the installer and run it. The extension will typically appear as a new tab on your Revit ribbon. Autodesk App Store (Plugins & Add-ons)