The simplest way to upload and download data is via the webinterface. Note however, that this interface is limited to DICOM and ECAT files. The project should be created before uploading data for that project. When upoading with the Java applet, explained below, a subject should also be created beforehand.
The interface can be reached on the website via the top menu -> Upload -> Images. After the correct project, subject and scan data is selected, the button 'Launch uploader' can be selected. A Java applet should appear (this can take a few seconds). A directory can be selected and uploaded via this Java applet. Complete step-by-step instructions can be found in the XNAT manual. If you have files in a zip file, they can be uploaded with the link after 'Looking for other ways to upload images? Click here.' in the bottom of the page. The result of uploading a zip-file is the same as uploading the extracted zip file with the Java applet.
See the FAQ if you receive an error while loading the Java applet.
All data goes by default to the prearchive. Here users can change the project and subject of an uploaded data sets before the data goes to the archive. All data that should be available on the website should be archived. The prearchive can be viewed using the top menu -> Upload -> Prearchive. A dataset can be archived by selecting a project and clicking archive. In newly opened window, users can check all information (project, subject, scans) and make any notes and/or alterations before moving the data to the archive.
Similar interfaces for download data exist as with upload data via the online webinterface. The data can be browsed using the menu left of the screen or by selecting a project on the main page. Once a session is shown with the scans, you can download the images via the right menu -> Download -> Download Images. Individual files can also be download with "Manage Files" in the same menu. The images (DICOM only) can also be viewed in a webviewer via the right menu -> View -> View images. More information can be found here and here.