To substitute unsalted butter for salted butter, add 1/4 teaspoon of salt for every 1/2 cup of butter. It is best to let butter come to room temperature and cream the salt into the butter before adding other ingredients.
Most recipes use unsalted butter because it allows the baker to control the salt content. Not adding the salt in baked goods means that the finished product will just be a little sweeter, which is okay when making cookies and cakes. It is not advisable to do so with pie crusts, as the addition of salt ensure the crust is not bland.