There are a number of steps to this and although this example is via YouTube, this will work with other media providers.
1. In YouTube, click "Share" on the video you want to embed
2. Select EMBED
3. Copy the iframe EMBED code
4. In your content on DesignSpark. Press the Insert Media Icon
5. Select EMBED
6. Paste the YouTube Code in the window
7. Press OK and you will see the video in the content