Using methods defined within .dll files in a C# Windows Form Application

Question

I have a .dll file which has many methods,interfaces and objects defined within it .. How can i use the methods defined within the dll file ?
When i define an object to any of the classes within it, it throws an error stating that cannot create object for an abstract class.
When  try to inherit , it asks me to define a set of frameworks within the abstract class ..
How do i proceed ?