Press B (by default) to open the Brick Selector Dialog. The blue spaces at the bottom are the bricks in your cart. Double-click the icons to add bricks to your cart. When you are finished, click "Done".
Equip Brick Edit
Press 1 to equip the brick tool. Use the mouse wheel to switch to different bricks. If you don't have a mouse wheel, you can use the arrow keys.
Shoot Ghost Brick Edit
Aim somewhere close and click the left mouse button. A transparent "ghost" version of the brick should appear.
Adjust Ghost Brick Edit
Move the ghost brick into position using the number pad. Numpad + and Numpad 5 move the ghost brick up and down.
Press Numpad Enter to plant the brick. The brick must either be resting on a flat surface, touching the top or bottom of another brick, or be embedded halfway into the terrain.
If a brick cannot be planted, an error message will flash on the screen describing why.
If you make a mistake, press CTRL + Z to undo the last brick you planted.