Workaround to Install Silverlight 4 on Mac OS X 10.6.8
I recently reverted from OS X 10.7 (Lion) to 10.6.8 (Snow Leopard) due to a number of Active Directory Service issues on my company's domain. As part of the reinstallation of Snow Leopard, I did a fresh install, which required that I also reinstall most of the plugins and helper apps that I had been using without a problem before I moved to Lion.
When it came time to install Silverlight 4 though, I ran into a major challenge. I downloaded the DMG file from Microsoft directly, but when I ran the Silverlight.pkg file, it would fail to install with a message:
Microsoft® Silverlight™ Browser Plug-In can’t be installed on this computer.
The plug-in requires MacOS X 10.4.8 or higher.
Now, I was positive that I was running MacOS X 10.6.8, and was a loss. After spending too much time searching the web, I discovered that there wasn't much in the way of installation help concerning my specific problem.
After some time I came across an unrelated forum comment that mentioned opening up the .PKG file directly within Terminal. While I'm semi-comfortable using the command line interface, I remembered that I could right-click on the PKG file and view the contents that way.
That gave me the information I needed to test out a hypothesis -- that the relevant files were already in a format that I could use and that the installer could be side-stepped. A few minutes later I was able to stream movies from Netflix again - hypothesis, correct.
Here are the steps needed to get Silverlight 4 installed on your Mac if the installer isn't behaving as it should:
(Optional, but recommended) Pour your self a stiff cocktail or open a beer. Take a drink to fortify yourself and reserve remaining.
Download the file, Silverlight.dmg from Microsoft's Silverlight site using the browser of your choice.
After it finishs the download, close the browser completely and quit - you'll be using it later, once you've installed the Silverlight plug-in.
Double-click on the file once it has finished the download to decompress it. You will now see a virtual drive called "Silverlight" on your desktop.
Right-click on the file, Silverlight.pkg and select the option for "Show Package Contents"
Open the folder labeled Contents
Inside you will find an archive labeled Archive.pax.gz, select it and then use the keyboard shortcut Command+C to copy the file (alternatively, right-click on the file and select Copy "Archive.pax.gz")
Paste the file on your Desktop (Command+V or right-click, "Paste Item")
Double-click on the file Archive.pax.gz and it will be expanded, leaving you with a folder labeled Library.
Once you've opened the Library folder, you will find two folders nested inside. You'll want to open up the folder called Internet Plug-ins
Copy the file, Silverlight.plugin that you will find inside. Once you've copied it, you'll be done with the Archive, so feel free to close the window.
Navigate to Macintosh HD (or whatever you may have renamed it) and then to Library  and finally, to the directory, Internet Plug-ins (path: ~/MacintoshHD/Library/Internet\ Plug-Ins/)
Paste the file, Silverlight.plugin into this directory
Open up your browser of choice and visit www.netflix.com to confirm that you can stream content.
Finish drink, congratulate self for working around something that you not have had to work around in the first place.
Good luck!