WorldWide Telescope is available for free download at WorldWide Telescope Website, . The Desktop client is only available for Windows PCs.
You can also use the WorldWide Telescope web client that has some of the same features as the Desktop version, but it does not require any installation. The Web Client runs on HTML 5, so you'll need a web browser that supports that.
If you have Mac OSX, you may also install the desktop version if you have also are running Windows on the Mac.
Detailed information and tutorials on how to install and use WWT may be found here.