An Ultimaker account is not required to download and install plugins from the Marketplace via Cura directly. Ultimaker Cura offers additional functionality provided by community developers through the Ultimaker Marketplace. Plugins are freely available and are stored locally on your computer.
Note: When visiting the marketplace.ultimaker.com webportal to install plugins directly into Cura, you will need an Ultimaker account to synchronize your changes.
A step by step guide on how to install plugins, can be found in the Ultimaker Marketplace article.