the molecular weight of MgCl2 is 95.21. how much MgCl2 would you need to make up 50 mL of a 1x10^-3 molar solution?
How many moles do you need in the final solution? That is M x L = ?
Then mols MgCl2 = grams MgCl2/molar mass MgCl2. You have moles and molar mass; solve for grams.