You can only upload content from your desktop, not your mobile. Head into the 'full brief' to find the upload button.
Can't find it? This could be for two reasons:
The deadline has passed and brief submission has closed. Please get in touch via the speech bubble in the bottom right-hand corner.
You haven't confirmed your participation yet. Select 'View full brief' and then 'Confirm' on the Full Brief page.
Once you've confirmed, the button should appear for you.