To use the Brush Tool, open the Library panel on the right and select the Brush icon. Select the desired assets to place or 'paint' in the scene by clicking them from the asset sub-categories. This will add them to the brush tool and they can be removed by clicking a second time. After the desired assets have been selected, place or 'paint' the assets by clicking in the 3D scene. Adjust the size and speed of placement using the sliders in the Brush Placement dialogue.
To remove assets click the 'Subtract Brush' option at the bottom of the dialogue.
Click 'Confirm' to confirm placement of assets before closing. The Brush tool can not be used to remove assets after confirming (you will need to remove using single select, rectangle select or the Organize tab).