In order to access the workshop and share or download machines with other players, you need to purchase the full version of the game. 

Once you are in a level or sandbox, you can click on this icon: 

You will need to be connected to a Mod.io account: you can either create one using your Google or Apple email address, or connect to your existing Mod.io account. 

Please note that it is not possible to upload or download machines from the PC or console versions of the game, as the mobile workshop is not cross-platform.