The white space may be caused by multiple, empty lines at the end of your first text capsule.
The add button is for you to use to add capsules (or boxes for text, video, photos, etc.). You can click on it where you want to add a box and then look to the right of the word "cancel". There are all of the capsule (box) choices for you to add to your hub. Just click on one and it will put that box in for you.
Good luck and I hope this helps!