Adding data to your Map Mavin account is as easy as dragging and dropping!
There are two ways to about it. Either drag files into the dotted-line box that appears after clicking the green Create Layer button or click within the box to browse through your own files. Bulk uploads are an option! Make sure that your files are zipped before uploading! Once the layer has been uploaded it will appear both under the Layers tab and as an option to be added whenever you are making web maps. Please note that Map Mavin currently supports zipped shapefiles, GeoTIFFs and GeoJSONs (zipped or unzipped), KMLs and KMZs, as well as WMS tile service URL's.
Update, September 27th 2018: In the Layers tab of the Admin panel, layer folders now have drop-down arrow icons beside them that make it easier to tell when they are opened (to view the individual layer files inside them) or closed.