How do I install Unity 3-D web player?

Their website says the plugin is only available for Windows and MacOS X:

Unity Web Player

The Unity Web Player is not currently compatible with the operating system that you appear to be using.

Unity Web Player for Windows Internet Explorer, Firefox, Chrome, Safari, Opera

Requirements: Windows XP/Vista/7

Unity Web Player for Mac OS X Safari, Firefox, Chrome

Requirements: Mac OS X 10.5 or newer

Your best option would be to install Windows in a VirtualBox and use it to view those sites.

Unity 4 will introduce Linux support, according to their website. Unfortunately, it will be a few months before that is released.

You can check out their webpage here. You can use CTRL F to launch the find utility, then type in "linux" to scroll down to that section