First, you must own a singer library. The original series of Vocaloid and Vocaloid 2 only come with singer libraries, but the Vocaloid 3 editor is sold as a stand alone, but will not work without a singer library. To install a Vocaloid, Vocaloid 2, or Vocaloid 3 singer library, the download or disc will have a setup.exe. Double click it to run it, and it will walk you through the process. You will need an activation key, downloaded versions usually have the activation key sent via e-mail, and disc versions should come with a pamphlet.
All libraries should be default in the singer library for their respective versions, but to use Vocaloid 2 libraries to Vocaloid 3 you will need to first install them with Vocaloid 2, then use ImportLib.exe which will walk you through importing the installed Vocaloid 2 voice to Vocaloid 3.