To access the drag and drop tab click the second icon on the left hand side:
From here you can drag any element onto the page. Just hover over the relevant item, hold down your left mouse button and drag the item onto the page:
When you drag an item over the page, containing elements like boxes and strips will be highlighted in yellow.
The horizontal pink dotted line indicates where the element will be placed when you release the mouse button.
You can drop Content Cards inside Containers but it is not possible to drop Containers inside other Containers.
Creating a New Column With Drag and Drop
If you have a Column Group on a page, you can create a new column by dragging a content card in between the existing columns.
The pink dotted line will turn vertical when your mouse is in a position between two columns.