As it said on detonationfilms.com, for white background stock footage fxhome product users were supposed to change the composite mode to multiply to make it transparent. The only problem is that when i go to change it, the only composite modes are add and screen. I've seen the multiply key on there once for something, but how do I make the mode go to multiply?
You can't use Multiply on stock footage in EffectsLab. You need to have CompositeLab or VisionLab.
Where you have seen multiply is on effects created by the program. Any effects created by the effects engines will offer a wide variety of composite modes. Advanced compositing of stock footage such as this is beyond the scope of EffectsLab's purpose, and falls in the realm of CompositeLab or VisionLab, as mentioned.