Type OBJECTPLACE ON in the console to turn it on, and OBJECTPLACE OFF to turn it off. SPEED <number> determines how fast you move up and down.
Controls:
Fire - Places the selected object at your location
Taunt - Removes the object at your location
Jump - Moves you up
Spin Dash - Moves you down
Camera Left - Goes to the previous item in the list
Camera Right - Goes to the next item in the list
NOTE: If you use Objectplace, and move to another level, you will be on item slot -1. Turn Objectplace off an on to fix it, restoring the normal non-gravity settings.