Thanks.
I followed your advice and did this:
1- In ButtonDown() call of tk2dUIDragItem_idn (my copy of tk2dUIDragItem), I stored startPosition via newWorldPos or offset
2- In the ButtonRelease, I tried to minus the end position from the start position via:
vector3 direcionToLaunch = transform.position - startPosition;
and then call addForce with this direcionToLaunch. But it doesn't work.
What I suspect is that I'm not getting correct position from tk2d. I saw that you do some world to camera translations and stuff like that and made me wonder if this standard approach of reducing endPosition from startPosition to get the movement vector might not work with the position you calculate and store in transform.position of game object.
So if you would tell me which positions I should use when I'm trying to do these calculations, I think that would help.
Thanks.