You have a concentration in micromoles. You want to deliver a particular number of nanograms. First convert micromolar (which is micromoles/liter) to nanograms/liter. To do that you will need the molecular mass of your primer. Watch out for the prefixes, it is easy to mess up the calculations by incorrectly converting micro to nano. Once you have converted micromolar to nanograms/liter, determine how many microliters you need to get your desired number of nanograms.