YouTube is a streaming service that doesn't provide a way to download videos from other users, so we don't have an import option for that.
If you want to add a video that you uploaded to your YouTube channel, they've got you covered with instructions on how to download your video here. If the YouTube video was created by another user, you'll want to reach out to that user to get a copy and permission to reuse their work. Once you have the hard copy, you can upload that to your Animoto project.