Centripetal is a force that draw something towards the centre - we usually experience it as the opposite, the centrifugal force (when you take a turn in a car, your body want to move straight on, you are pushed to the side of the car. The force that the car exerts of you to make sure you make the turn together with it is the centripetal force.)
Normalizing power is a force that normalizes things, that is, it sets a norm and makes sure that it is observed.
Normally, a state has these powers: a state has the ability to create a norm to be observed by the people living in it (this is usually laid down in the law) and a state "draws" people towards it's centre, that is to say, it makes people feel they belong there, it attracts them. So even someone living near the border of a normal state will feel they belong to it (someone living near the German border in the Netherlands is likely to still feel a strong sense of being Dutch, living in the Netherlands).
Macedonia, however, lacks these powers for the reasons mentioned in the sentence. It contains many different (ethnic) groups with different cultural, linguistic and religious backgrounds (heterogeneity), it is a complex society (difficult to create one set of rules or norms!) and the people living there have different ways of identifying themselves with the country they live in (they may, in this case, tend to think of themselves as Turks, Greeks, Slaves - rather than "Macedonian").
This results in Macedonia being a great example of what the author describes as "borderlands": an area where central government cannot wield the powers it normally has, to unite the people in custom and law (normalizing), and to create unity among them (centripetal).