The Grease Pencil: Design add-on’s features can be found in three places: a panel in the 3D view, the Grease Pencil Point Menu, and the ‘Layers’ context menu of the Grease Pencil Dope
Design Panel
By default, the Panel’s name will be ‘Design’, however this can be changed in the Add-ons Preferences (Edit > Preferences > Add-ons > Grease Pencil: Design > Panel Category)
In order to see the features of the Grease Pencil: Design panel, you must be looking through a Scene Camera. The reason for this is because, though many of the design features work in 3D, the features are meant as 2D Design features. Grease Pencil Points, Strokes, and Layers can be Aligned, Distributed, Arranged, etc. to the Selection or the Camera’s View.
Point Menu
When the active object is a Grease Pencil object and the Mode is set to “Edit Mode”, you will see a Point Menu (see image, above). In this menu are several new commands added by Grease Pencil: Design. More about those can be found in the Bevel and Merge Points sections, respectively.
Layer Context Menu
Grease Pencil: Design adds several powerful features to the Grease Pencil Dope Sheet Context Menu. You can find these features under ‘Layers’ (see image, above).