Linking to specific libs ...

The task says to add that particular .lib to the list of libraries that get linked to your code. Without saying that this library should be linked, the implementation for the functions defined in its headers is not available to the linker and you get that unresolved external symbol.

In VS, you can add something to the linked libraries list either through a #pragma comment or in the project settings:

// at the top of main.cpp, preferrably
#pragma comment(lib, "the_lib_name.lib") // .lib optional

You can have different libraries for debug and release with simply surrounding the #pragma commentin an #if block:

#ifdef NDEBUG // release
#pragma comment(lib, "the_lib_name.lib")
#else // debug
#pragma comment(lib, "the_lib_named.lib") // most debug libraries end with a 'd'
#endif

And for the project settings you can do so with

[Project] -> <Project Name> Properties (or Alt-F7) -> Configuration Properties
    -> Linker -> Input -> Additional Dependencies

Just add the_lib_name.lib at the front (followed by either a space or a semi-colon ;). Make sure you add the correct library for the active project configuration (debug / release).