If you land on this post and you are looking some simple rigidbody actions for grenade/cannon ball launching, see my post here:
http://hutonggames.com/playmakerforum/index.php?topic=15323.msg70845;topicseen#msg70845 (also for path prediction on screen).
While the above asset I posted in this thread is indeed great (and my actions are working!), they can be a bit complicated if you just need 1 simple thing. Of course, if you are looking to make various ballistic calculations for projectiles, then the package in this thread is top notch.
