There are two variants of adding a new element to your design:
To add a new element:

This way allows adding a new element by using the navigation buttons of Components tree.
To add a new element of the same nesting level as the currently selected one - click the button
.

To add a new element of the lower nesting level to the currently selected one (to nest an element) - click
.

There is the only way to delete an existing element. It consists in the following:
Select an element you wish to delete in the Components tree;
From the top bar of the Components tree click the
button;

After clicking the button a confirm window will appear;

In the confirm window click OK to confirm the operation or Cancel to cancel it.