To add objects is one of the easiest things in the Worldbuilder (for me). Just press the circled tool. !!!CINEMATICS ARE NOT PLAYABLED!!! Its just to make a Map Intro.
Objects are only at ByNativeType and REGION. Just choose some thing you want and place it.
Some times you have to place the objects deeper or higher for that make the Z: bock more as 0 or smaller.
DON'T FORGETT AT PLACING GATES TO GIVE IT TO THE RIGHT PLAYER.