To install a Dialogic, download it as a ZIP archive. All releases are listed here: releases. Then extract the ZIP archive and move the
addons/ folder it contains into your project folder. Then, enable the plugin in project settings.
If you want to know more about installing plugins you can read the official documentation page.
You can also install Dialogic using the AssetLib tab in the editor, but the version here will not be the latest one available since it takes some time for it to be approved.