Firstly, please provide descriptive titles for your threads. We should be able to have a fairly good idea of what a thread is about simply from the title. Your title doesn't really provide much in that regard.
As for the question, you don't really "open" a DLL. Please provide a FULL and CLEAR description of what you're trying to do, how you're trying to do it and what actually happens when you try.
A DLL is a library of types and/or functions. As a C# developer, you are generally in the business of accessing the DLL to use those types and functions. If it's a managed (i.e. .NET) DLL then that means adding a reference to that DLL in your project, which you do in the Solution Explorer. If it's an unmanaged DLL, e.g. one written in C/C++ then you will need to use Platform Invoke (pinvoke) to access it, which is the same way you invoke the Windows API. If that's what you need then we can go into more detail when you have confirmed that.