To fear God is a positive command. Even though it is not included in the Ten Commandments, it still is one of the many other commandments that are in the Old Testament. And one of the original sages of Musar, Rav Isaac Blazer shows in his book "Or Israel" that it is the key to everything, all good and all holiness. [I have not read that book for some time, but the point was clear.]
However the Musar movement has a slightly different approach to this than what I have seen in the Rishonim.
Most Rishonim go with the idea that to contemplate the works of God brings one to love and fear God.
This is explained in the Guide for the Perplexed that it means learning Physics and Metaphysics. [That is what the Rambam says in a round about way. You have to put it all together. Start from the Introduction where he defines the work of Creation and the Divine Chariot as means the subjects of Physics and Metaphysics as the ancient Greeks understood them. Then later in the Guide he says that by learning the work of creation one comes to fear of God and by learning Metaphysics one comes to love God.
But this is not apparent because what people say they believe is different from what they actually believe. You can tell what a person actually believes based on his or her deeds, not their words. people with real fear and love of God do not look religious and make a point of keeping their love and fear of God internal.
They do not make a public show of their fear of God because they value it.
But what I would like to add here is that the purpose of this learning ought to be directed towards Fear of God and not towards understanding. So the way to go about this learning is to direct one's attention to love and fear of God, not towards getting good grades of making a living.
However to get back to the original point of Israel Salanter and Isaac Blazer, I would say that they are right about the emphasis on Musar in terms of orientation. That is to get a proper idea if what Torah is all about is only through the Musar of the Rishonim and Achronim. So the first step to coming to fear of God, I think is in fact just as Rav Israel Salanter thought--by an emphasis on the classical books of Ethics of the Middle Ages. And after that, the Physics and metaphysics.
[But I must add that Physics means real Physics, not layman's books. Metaphysics books means as the Greeks understood it as Ibn Pakuda [author of the Chovot Levavot] and the Rambam make clear.]
