If you have downloaded our SDK, you will need to create an account within our editor pack to be able to upload content. You can see how to install the editor pack here if it is not installed already.
Once it is installed, to make your Creator account, click on the 'Sinespace' button on the Unity top bar then 'Sign In'.
In the Inspector, you will see the Sinespace Login Information, along with a button 'Create an account'. Click on this button, and it will take you to the same Creator page mentioned here.