You can add assets (i.e video or images), via the asset tab on the left. Choose between "Select a single asset" and "Autofill all groups".
Select a single asset
You’ll automatically be shown images or search terms, in the asset browser, based on the text you wrote in the block.
Click on the image or images you want to add to your asset library for this story. Once they’re imported click on one to add it. If you want to use the same asset across more than one block or remove an asset from an adjoining block you can use the corresponding arrows to do so.
You can apply one asset per block in the script editor. But if you have more than one slide in a block, and want to use different assets for each, you can do so once you open the storyboard in the video studio.
Autofill all groups
Assets from the asset library will automatically be added to all your asset groups. If it's not enough assets in the library, the autofill will fail. Then you can always add assets to your project library, and try the function again. If the "include Stock Assets & Music" is toggled on, it applies stock media for you in your script if the selected library is empty or has too few assets.