You can upload your media from the 'Storyboard' screen, 'Edit Scene' modal or the 'Advanced Editing' screen using the following process:
Step 1: Go to the 'Media' tab on the left panel.
Step 2: Click 'Upload' at the bottom of your screen.
Step 3: Select the files you want to upload and click 'Open.'
Step 4: Access the files you just uploaded under the ‘My Uploads’ section.
Before uploading, make sure your file meets the following specifications:
Format: MP4, MOV, JPG, GIF, PNG, MP3, AAC, WAV
Size: Ideally Not More Than 800MBs.
Here is a short video explaining the process.
Also, the speed of uploading media is highly dependent on the bandwidth of your internet connection. Please ensure you are on a good internet connection while uploading heavy files.