1. Check that your Array is public and non-static.
2. Place the [System.Serializable] attribute before the arrayed class declaration
3. If your arrayed class inherits from Object, don't write it (that is: "public class MyClass:Object" is wrong, "public class MyClass" is right). Don't know exactly why, but I had the same issue, and finally realized that removing the Object inheritance made my class being serialized (I was using a List instead of an Array, but I suppose it's the same).

Sample serialized Class:

[System.Serializable]
public class MyClass
{
   // Class stuff
}