如何在 VisualStudio 中向项目添加. dll 引用

You probably are looking for AddReference dialog accessible from Project Context Menu (right click..)

From there you can reference dll's, after which you can reference namespaces that you need in your code.

Copy the downloaded DLL file in a custom folder on your dev drive, then add the reference to your project using the Browse button in the Add Reference dialog.
Be sure that the new reference has the Copy Local = True.
The Add Reference dialog could be opened right-clicking on the References item in your project in Solution Explorer

UPDATE AFTER SOME YEARS
At the present time the best way to resolve all those problems is through the
Manage NuGet packages menu command of Visual Studio 2017/2019.
You can right click on the References node of your project and select that command. From the Browse tab search for the library you want to use in the NuGet repository, click on the item if found and then Install it. (Of course you need to have a package for that DLL and this is not guaranteed to exist)

Read about NuGet here

Another method is by using the menu within visual studio. Project -> Add Reference... I recommend copying the needed .dll to your resource folder, or local project folder.

For Visual Studio 2019 you may not find Project -> Add Reference option. Use Project -> Add Project Reference. Then in dialog window navigate to Browse tab and use Browse to find and attach your dll.