sorry for the stupid question but I cannot find a way......
Any preset you make either on your device or on the amp, generally your device because it's much easier to do, will become part of your Library once it is saved. When your device is connected to your PC, and MyMarshall is running, and you have selected the My Gateway tab, this new preset should be listed. (You may already know this so my aplogizes if I'm stating the obvious).
User created presets / tones are published (uploaded) from My Gateway. Left click on the gear icon and select publish. This specific procedure is not part of a tutorial or a manual, that I can find, so using a little intuition based on other application use helps find the command / process. Don't be afraid to do some exploration of the site. Most of the answers are there, believe it or not.
Hope this helps.