Actually, you have already found your grandfather's father - Alexander Sandy Mcdonald (Mcdonald-3575) In order to find your grandfather's mother, you need to search for Alexander's marriage.
It would also be helpful if you put in a few locations. Are these Scotland? You need to add more details to the people contained within your tree.
I would also suggest you return to your question and click on the small link 'edit' and add the tag scotland to attract their attention.