The Source Code button in the toolbar looks like <> and is to the left of the Insert Snippet button.
If you need to embed code into a page, like a Google Form or a Flickr gallery, you would do the following:
1. Copy the embed code from the website or source (e.g, Google or Flickr)
2. Open the page on your site that you want to edit by clicking the Edit Main Content or Edit Promo Section button
3. Click on the Source Code button in the toolbar
4. You will see the source code of the editable area of your webpage. Find the place on the page where you want to embed the code
5. Place your cursor at the front of the line where you want to embed the code and paste the code there
6. Save the page and publish when ready