How to add a new primative.

First you need to create a new object where your 3D shape is going to be placed. To do so:

1. Open the Objects List by clicking the objects list icon in the lower left corner of the workspace (shortcut shift+o). 

2. Click + New Object and a new item will appear in the Objects List. When you create a new object, your workspace will automatically switch to Edit mode.

3. Once you’re in Edit mode, you can choose from the 3D modeling toolbar a few tools which create primitive objects such as box, sphere, cylinder, tube, cone, polyhedron or torus. Draw your own mesh shapes with the Draw tool. 

You can edit a mesh model in Edit mode. To get into the Edit mode of an already existing object, you first need to select it. Double click on the object on the scene to get into Edit mode. You will see the mesh structure of the 3D model and a 3D modeling toolbar at the bottom of your workspace.