http://msdn.microsoft.com/en-us/library/7314433t(v=vs.80).aspx
From MSDN:
- In Solution Explorer, select the project.
- On the Project menu, choose Add Reference.
- The Add Reference dialog box opens.
- Select the tab indicating the type of component you want to
reference. - In the top pane, select the component you want to reference, and
then click the Select button. Press CTRL while clicking to select
multiple components.