Prednizone causes several effects that all result in weight gain. For me, it was uncontrollable appetite, which also followed a long 2 months where I barely ate more than 300-400 calories a day.
As far as salt, I only know of hypertension issues that steroids tend to aggravate. Sodium (in salt) is necessary along with many other simple elements. The daily recommended value by the FDA is pretty high, but prepared/frozen foods and fast food use it to preserve and flavor food - salt is cheaper than other options for preservation or flavoring.
You can reduce your sodium levels by eating potassium-rich foods (potassium replaces sodium in most compounds - high school chemistry class). Bananas, raisins, and other fresh fruits and vegies are good sources. Skim milk and some yogurts can also provide some.