Simply tell the actor you want to drag around that whenever it is touched it's position should be the same as the position of the mouse.
So the rules would be saying that: Whenever the actor is touched . . . . . Constrain Attribute - Actor's position X to Mouse's position X Constrain Attribute - Actor's position Y to Mouse's position Y
Comments
So the rules would be saying that:
Whenever the actor is touched . . . . .
Constrain Attribute - Actor's position X to Mouse's position X
Constrain Attribute - Actor's position Y to Mouse's position Y
Like this:
http://img842.imageshack.us/img842/9877/screenshot20111211at142.png
Do when touch is pressed, constrain actor position X to mouse position X
AND constrain actor position Y to mouse position Y
Edit: Tynan, beat me to it by 10ms.