Log in to your Avid account and navigate to "View My Products." Select "Media Composer Ultimate" to find the download links for your operating system (Windows or macOS).
The download process depends on whether you are trying a free trial or activating a paid subscription.
Unlike the standard version, includes a bundle of high-end software options designed for collaborative environments: Media Composer 2019 FAQ - Knowledge Base
You can access a fully-featured 30-day trial of Media Composer Ultimate by creating an Avid account and providing your details. A download link is typically emailed to you.