Do you want to upload or download aircraft? If you want to download aircraft from the workshop just go to the workshop and subscribe to the ones that you want (green subscribe buttons). When you load the game the next time you should go to the modding menu option on the opening screen and put a green checkmark on the mods that you subscribed too.
If you have any further questions assistance fill free to DM me.
you can also find great help at these links on how to use steam and download/upload native/workshop aircraft as well as create your own airlines (native).