this is normal. rivel won't use her cat tree until 2 months it's been sitting in the house.
1st reason could be the smell. you know that "new thing" smell? it just might be unpleasant for cats. just keep playing with them on it, you might try to rub your hands on the scratcher or carpet too so it have your smell. eventually the factory smell will be replaced by your house's and family smell and it might entice the kittens to play on it.
2 is location. where do you put it? i find cat tree is most optimal behind a window where they can peek outside, but also overlook their "main" room, the room they hang out the most where they can watch family activities.
Thanks for the videos Apoloo, I'll watch them this evening... you bring up a good point about the cat tree, I may need to move it more towards the middle of the room or even the large set of windows. I *did* find that one of them enjoyed chilling on the mid level scoop!