I followed along the demonstration video and made my own, slightly different layout, which required nested horizontal and verrical groupings. I found that the design mode had severe problems letting me navigate and select the components. I don't expect you to fix the tools, but for the purposes of the video, mentioning the limitations of the design tool would be a good idea. I resolved the selection problems by ignoring the design window and moving the text in the source window.
Another point that caused major confusion is that you seem to get different controls depending on if you drop the components inside a form or outside. For example if you drag a "DropDownList" into a form, you get a mx:FormItem, wheieas if you drop the same item outside the form you get just a s:DropDownList. This caused major head scratching when I accidentally dropped one outside the form, and tried to continue by picking it up and dragging it inside.