The Trade Now button can be used to add or remove securities from your portfolio. While on your Playground, click on an item once and select Trade Now on the Action Menu.
While inside any Fund or Company Snapshot, you can select an option under the Trade Now drop-down menu near the top left corner.
You can also trade while inside your Portfolio Snapshot. Click the Trade Now button on the header of the Snapshot, or choose an Item from the "Holdings" tab at the top of the Snapshot, click once on a Holding, and then press the "Buy" or "Sell" button -- or simply click on the Item itself and select Trade Now from the Action Menu.