What does "the magnesium salt of EDTA" mean?
The authors have some strange kind of humor... it's a riddle.

Below you might see


  2) If the magnesium salt of EDTA is unavailable, dissolve 1.179 g disodium salt of
  ethylenediaminetetraacetic acid dihydrate (analytical reagent grade) and 780 mg magnesium
  sulfate (MgSO4⋅7H2O) or 644 mg magnesium chloride (MgCl2⋅6H2O) in 50 mL distilled water.
  Add this solution to 16.9 g NH4Cl and 143 mL conc NH4OH with mixing and dilute to 250 mL
  with distilled water.


From the molar mass of $\ce{MgSO4 * 7 H2O}$ ($\pu{246.47 g/mol}$) you can calculate the number of moles of magnesium the "magnesium salt of EDTA" should contain. With the help of the data sheet of your purchased magnesium salt of EDTA you can calculate the required quantity in grams.

You also could calculate possible stoichiometries of the imaginary "magnesium salt of EDTA" if you like. But be aware that not all commercially available magnesium salts of EDTA have a well defined stoichiometry.