To install right click on "dopplerender_v.X.X.X.zip" file right below THIS description > Save Link As to download it, do NOT unzip it. In Blender Edit > Preferences > Add-Ons > Install > in the file explorer find "copplerender_v.X.X.X.zip" and select the zip file pressing Select > in the Add-Ons search search for dopplerender and enable the Add On to complete the installation.