You can add/create a new activity by one of 3 followings methods:
Using Left Toolbar
Hover over the left Toolbar and select the top icon (Activity) from the Toolbar.
Click on the icon, drag it to the desired spot on the canvas, and release the mouse button. An activity will be created in that spot.
Double-click on the Canvas
Double-click with the left mouse button on an empty spot on the Canvas to create a new activity
Right-click on the Canvas
Right-click on your mouse and select Add New Activity.
Note: Learn how to create a Chain of Activities here
PRO TIP:
Hit the Tab key after entering the name of the activity to jump to the duration field.