Before adding content to your email you must first add Rows to create your desired layout.
To get started select on the Rows tab:
You can now drag and drop different row layouts to your email design.
On the Content tab you can drag and drop the content types you would like to populate your rows with:
You can duplicate or delete the row by clicking on the outer area:
On the left-hand side, you can drag the row in order to change its position within your email: