If you are trying to embed an iFrame for a video share, you cannot do this with the iFrame embed code provided by YouTube.*
Instead, click the Add Media button from your content (text box, other module), then click Insert from URL and paste in your URL. The first screenshot below shows the Add Media button from a text box.
The screenshot below shows what happens after you click the Add Media button and paste in your video URL. Click insert into post, and a video player with your video will be added to the page/post.
*Because Big Tomato runs a configuration of WordPress called 'MultiSite', which is a network of connected websites, site administrators cannot add Javascript or iFrames to pages/posts. This is a security matter because allowing for site admins to add unfiltered code is a big risk of breaking the whole network.
We get many benefits from using a network configuration, including the ability to share plugins, security measures, user accounts, content, and other elements, which makes us more efficient and keeps our pricing low.