Assuming you are developing a Windows Store App, there is a limited subset of the .NET framework that is available for your use. The assembly you are using is evidently built on a full .NET framework and is not a valid Windows Store App component.
If you have the source code for financial.dll
you can try to port it to the sandboxed version of .NET available for WinRT. If you don't have the source code, you can't use the assembly at all and will need to find an alternative. In this case, perhaps you can edit your question to specify which financial functions you need and perhaps we can suggest alternatives for you.
EDIT
For the functions you listed, there's a C/C++ source code list here.