I wouldn't exactly say that was a curved path....its two separate movements..once they start moving they touch the score HUD then move down to the other box HUD...
Tweening can be done ..even manually like you attempted..you were on the right track for manual tweening.
Try 'moving' your object to the co ordinates but stop short by say 20 or 30 pixels, then do some micro adjustments to close the gap..
Also those butterflies look like they have a slight 'sine' function on them for added movement variation...both horizontal and verticals ..
Try adding those behaviors also and toggling their values at each movement call...
I use sine behaviors for nearly all butterfly or movement tweening ..if you tweak it just right you can get some fantastic smooth movements and random variations....:-) :-) hope that helps...