The download is available on connect.microsoft.com. You'll have to get a "liveID" using your email address, if you haven't gotten that already. The download process has 3 steps:
The entire process took me about 10 minutes not including the download time. The Getting started guide included in the HPC Pack is a step by step guide for installation and node deployment. Of course, you will have to have windows 2008 server to run the HPC pack. The server standard trial can be downloaded from this link.