You have to make an account. (It's free)
When you log into your account, go to "My files", then you should see an "Upload" icon at the top, just click it and drag whatever you want to upload in there.
If you want to upload maps, you should probably clear your Local Disk - FeralHeart - exports folder temporarily. (It's easier that way) And once you've exported your map in game you need to go to the exports folder and select all the files you have in there, then right click and choose "Send to", then pick Compressed (zipped) folder. When you've done that you need to drag that folder to mediafire and upload it.
If you want to upload presets, you should clear your Feralheart - presets folder temporarily. Now export your preset in game, once you've done that a file will appear in your presets folder, now just drag it into mediafire and upload.
I hope this covered all your questions. c:
