I have searched in so many places but I couldn't find where is Brainbox at? Is it inside Buildbox? Or is it a separate software?
Click on the character or objects that is displayed one the left side panel.When the item is selected look on bottom right.You will find +Add Brainbox.
Take a look at the screenshot. Click on 3d world and single click the character/object on left and look out for the addbrainbox on the right.