The editor provides a tool to preview, edit and add materials.
To access the tool, just click on the toolbar View -> Materials Viewer....
To add a new material, just select the tool's toolbar Add.... A dialog will appear to select the type of material to add. Once selected and confirmed, the material instance will be added to the scene. You can edit its properties and assign to meshes.
To edit a material, just click on the desired material to edit. Once clicked, the Editor's Inspector will be updated and show up the material's properties.
To assign a material to a mesh in the scene, you can drag'n'drop the material on the mesh. Or, change the material value in the Inspector once you have selected a mesh.
Here is a live demo using the Materials Viewer tool: