The HTML widget is on the right-hand sidebar, between the "two-column" and "file" widgets. When you add it to your page, an HTML editor will open where you can paste your HTML.

If you're having trouble finding it, it may be because the the sidebar is showing Properties at the moment. As you can see in the picture on the right, there is a "Content" tab and and a "Properties" tab at the top of the sidebar. If you can't see the HTML widget, try clicking on the word "Content" so the "Content" tab becomes visible.

Another reason you may not be able to see the HTML widget is because you are in one of the content folders, like "Photo Galleries" or "Video". When you're in one of those folders in the sidebar, you'll need to click the blue "Back" button in the sidebar (just below the word "Content") to bring you back to the view where you can see the HTML widget.

If you still cannot see the sidebar, then it may be hidden. In that case, click the "Show Content Sidebar" in the top right corner to see the sidebar.