Automated unit testing for third party application

We do customization in Findur a product from Open Link Financials Ltd  using C# .Net 3.5 (VS 2008) and it produces a dll assembly.  What we need to do is to start automated unit testing using VS test project/NUnit.

 

Challenge –

1)So far, we have tried mocking the class objects by creating a mock class so that we can test it using NUnit. But NUnit throws an error because of the references of open link dll’s in your test project.

2)We tried to serialize the class objects to create a mock object, but we are not able to do it so as the class cannot be serialized.

 

if any one of you can help us to find a way to unit test these assemblies.