First off, the plans are in PDF format, and you'll need some sort of PDF reader. I use Adobe's Acrobat Reader. If you can't read/open PDFs, then you'll need to get that.
So, first you'll need to get to the page with the download. The PDF is hosted on Google Docs. Get there by going to the Plans page, and then clicking on the link "Water Rack Storage Plans"
That should then take you to a page that looks like the following:
On that page, there is a "Download link"
Upon clicking the "Download" link, you will be asked to save the file locally. A dialog similar to the following will appear (depending on your browser and operating system):
At that point, you may either choose to save a local copy of the document, or open it. You now have the plans. Happy constructing!