If i remember correctly the object makes it really simple to move active objects around as if they're on a 3D field instead of 3D. Like in Mario Kart or other old '3d' games.
Well, if it's anything like MMF 1.5's 3D sprite object, I would assume it's just a way to display 3D models in your game. (I never used MMF 1.5, but I've gathered that's what it did) You could use them instead of pixel art. Doing a full 3D game with them is probably not possible, or at least not very practical.
3D Actives has nothing to do with importing 3D meshes. Like Nick said, what it does is scales 2D sprites to look like they're on a 3D plane, kinda like what Doom did with its enemies back in the day. The 3D Mesh object imports 3D meshes, however it is quite buggy and doesn't have as much flexibility as you might hope.
No but i could type 3d active in the article search and see what comes up. Or I could just look at the examples that came with the extension. Actually, I would those first before I start asking other people about it. That's why i gave you the non-answer above. The best way to figure something out is to play around with it.