You can customize your media collection from the Global Options/Media menu.
The Media Tree shows the root folder / and any other files or folders you've added. Folders will be blue, and normal files green. You must select a file or folder to use the other options.
The New Folder button requires a parent directory (blue node) to be selected and highlighted. It allows you to create a new folder inside that one.
The Upload button is how you contribute new media files to a folder, so it also requires selecting a folder first. A 100k limit is placed on files.
The Delete button allows you to remove the selected file or folder. Selected folders will remove all nodes inside them.
To use your custom media inside MXP or MSP, you must use the "media://" prefix, followed by the path of the file. For example, if you have the image file "merch.jpg" inside folder "myimages", then the full url would be "media://myimages/merch.jpg."