I haven't done this workout in a while, but if I remember correctly, the explosive lunges are done first with 3 (?) static lunges, then you "explode" to switch which leg is in front. I assume it's the change that is the tough part (and the end where you do them back-to-back). This is a challenging move, but you can get better at it (isn't it great to have a challenge to work up to rather than be bored to tears by a "too easy" workout?).
Start with either a more limited range of motion, a slower pace, or both. That's the way I approach moves I want to do but am not quite ready for (some moves I don't EVER want to do, so I modify them to suit me).
You can also do the entire explosive part with the same leg in front, so you're only concentrating on the power and height at first. Or, conversely, concentrate on the foot switch and do a very shallow lunge (almost like a scissor move).