DLL library for C# Console application in Visual Studio

Question

Hello,
I just received a third party .dll (library) file to work with as a task to complete in C# on Visual Studio app.
I am supposed to create a program that decrypts input data and get output based on a .dll file (written in C++)
But when I am trying to add and use .dll file in Visual Studio i always get an error that says:
"A reference to (file location) could not be added. Please make sure that the file is accessible, and that it is a valid assembly or COM component"

Please help me how to add and work with .dll file.