Is your mouse pick distance enough to actually hit the terrain? Check if the gameObject yout pick hits is null, if it is, it's not actually colliding and then it'll return the vector (0/0/0) .
Anyways, tween is a fancy unity term for interpolate based on a function (curve). For example, if you tween linearily and have your actor at (0/0/0) and your target at (1/0/2) and want to move between them within 1 second, then your actor is going to be set to the position (0.25/0/0.5) after 0.25 seconds, to (0.5/0/1) after half a second , and to the target after 1 second (well, it does this every frame, so it will seem smooth).
So no matter what, your colliders will not do anything, since they haven't noticed any movement. The actor just happens to change its position.
So while tweening can be useful, this is not one of its' strongsuits.
To make unity colliders (and the character controller really is just a special kind of collider) register the movement and check if that movement is actually possible, you need to use actions like "move" or "simple move". These are especially done for this purpose only.
After making sure that your pick hits and that you use an action that takes collisions into account, you need to think about how far you want to go with your controller. Eg :
p= player
x = goal
I = wall
Imagine they are positioned like this : p I x
Then if p moves towards x, it will get stuck : pI x
This would require the player to carefully click the actor around the wall, so that it does not collide with the wall.
To solve this problem you
need Pathfinding, there's no proper way around it. Unity pro has such a system integrated, though it's too young to be used in any real production imo. That leaves you with a couple of plugins most of which cost money and of which all but one are not yet supported by PlayMaker(to my knowledge).
So without advertising my own little project here, I don't think there's much alternatives but Aron Granberg's Pathfinding solution (
http://arongranberg.com/astar/ ) coupled with the Playmaker integration which can be found here :
http://hutonggames.com/playmakerforum/index.php?topic=2540.0[Don't worry, the free version of Astar is more than enough for your purposes]