An assembly can be attached before or after doing the takeoff. Click on the takeoff name to get to the sub-menu, and then click Edit.
At the bottom of the screen will be an Items and Assemblies section ( this is also viewable when creating a takeoff). Click the Add button
The folders contain the assemblies. Click to expand the folder and sub-folders. You can attach one assembly or multiple assemblies by clicking the + to the right of the assembly.
Then click the Add Assembly button, and answer the questions to yield the materials
TIP: The end of each assembly name shows the measurement type. This measurement type must match the measurement type of the takeoff (chosen from the drop-down when creating the takeoff).