MgCl2 is very hygroscopic. This means it really likes to be in water and it will pull moisture from the atmosphere so it can be in it. For this reason, we use a 1 M solution of MgCl2 already in water supplied to us from Sigma. This way we do not have to fuss around with trying to weigh out an appropriate amount of MgCl2, since what we weigh would also include water.
MgCl2 should be stored in its original container at room temperature. You do not have to desiccate it since the MgCl2 is in solution already.
Finally, never put a manual pipetter into the magnesium chloride container. Always take some out of it with a sterile, individually wrapped aspirating pipet and transfer it to a sterile tube/vial.