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Right click on "References" in the solution explorer and click on "Add Reference" as shown below in the image.

in Visual Studio, go to Solition Exporer, expand your project, right click in a folder called "References" and click "Add References", browse the references (assemblies) until you find the one you want. If you're using VS 2010, you can simply type "system.numerics" on the search up top. Then just select the reference and click Add, then Close.