ppm is a mass fraction. it is similar to percentage where u multiply the fraction by 100%. In ppm case, u multiply by 1 million.

ppm of a solute = (mass of solute) / (mass of solvent) x 10^{6}

37ppm of Mg tells u every million kg of solvent corresponds to 37kg of Mg. Given this information, you should above to work out its molar concentration by converting the required mass to moles.