A few questions:
Does your project include 1 top-level collection or 2 top-level collections? Select "Collections" in the left rail, and then look for either one collection called "default" or two collections called "default phone" and "default tablet." If you just have a "default" collection, it means all your content will appear on both phones and tablets. If you have both "default phone" and "default tablet," you need to specify content for the tablet collection. This is determined by a project setting that you specify when creating the project. At this time, you cannot edit this setting.
Do you want to include different content for tablets and phones, or do you want to use the same content for both? As a general rule, HTML articles work well on both phones and tablets, but for fixed-layout (InDesign-based) articles, you might want to consider creating separate articles for phones and tablets.
Next week's release includes an exciting new feature that we're calling "layout renditions." When you define a layout template, you'll be able to create different layouts (or renditions) for tablets, phones, and the web. For any collection that appears in both phones and tablets, you'll be able to design different browse page layouts for the target device.