Why there is no change in water level when salt is added?
This is an example of electrostriction - see, for example, this paper for a full explanation. 

In short, the solvent molecules become more ordered in the vicinity of dissolved (charged) ions and less ordered as we go further away from the dissolved (charged) ions. The increase of solvent density near the ions is offset by the decrease of bulk density in the solvent as distance from the ions increases. This results in an overall net reduction in the observed volume of the solution.

From the Discussion section of the manuscript:


  The interaction of the electrostatic field of an 
  ion with water tends to align the dipolar water 
  molecules in the direction of the field. In this way 
  the field tends to disrupt hydrogen bonded 
  structures in liquid water, and to compress the 
  water molecules surrounding an ion. These 
  electrostatic effect give rise to a shrinkage of the 
  water.