To create an instance of a class, it must be public and have public constructors. If the classB has only a custom constructor with multiple parameters, you can instantiate it like this:

var = MyLib.ClassB(x, y);

for the second issue, the + means eResults is a nested class of ClassA. You can't instantiante directly, but there is a workaround based on reflection described here: Working With Nested Classes:

a = NET.addAssembly('c:\myClasses.dll');
t = a.AssemblyHandle.GetType('MyLib.ClassA+eResults');
var = System.Activator.CreateInstance(t);