You don't add or link directly against a DLL, you link against the LIB produced by the DLL.
A LIB provides symbols and other necessary data to either include a library in your code (static linking) or refer to the DLL (dynamic linking).
To link against a LIB, you need to add it to the project Properties -> Linker -> Input -> Additional Dependencies list. All LIB files here will be used in linking. You can also use a pragma like so:
#pragma comment(lib, "dll.lib")
With static linking, the code is included in your executable and there are no runtime dependencies. Dynamic linking requires a DLL with matching name and symbols be available within the search path (which is not just the path or system directory).