While creating a world, select Props, once you've gone down to the 'Props' tab, you need to select the 'Edit' icon to bring up the scroll list at the bottom of the screen. From here, press UP (up triangle??) on the D-Pad and this will bring up the menu where you can find an Assembly.
EDIT: Some other clarification: highlight the row in the top left corner of the screen and move the cursor all the way to the right. Assemblies is the last category in that row.
Just select one and position it the canvas. After a few moments, the cheev will pop.