XNAT Marketplace makes getting your plugins and tools easy. Browse or Search to find what you need and click on any of the "Download Now" buttons. This will copy the plugin or tool to your local machine. Once there, check the add-on's documentation to learn how to use the tool.
The "Download Now" buttons act like any other "save link as" function in your web browser. Typically, files will be saved in your Downloads folder. However, some browsers will remember the last folder to which you saved a file. If you cannot find your add-on, check your download history.