WWT is free for use by anyone. To download and install Windows desktop client go to worldwidetelescope.org.
WorldWide Telescope for Mac OSX
To use the desktop client on a Mac requires the Windows operating system. On a Mac, you need Microsoft® Windows 7 (minimum), Windows® 8 (recommended) license installed on your Mac via Bootcamp, VMWare Fusion, or Parallels. For more help and information about installing a Windows Operating System on a Mac, see this page: Installing Windows OS on a Mac.
If you do not want or do not have admin permissions to install the program on your computer. A second option is the WWT web client that runs in a web browser.
For more information see: Worldwide Telescope Client Feature Comparison