So why can't my QT version 10.0 (90.3) play h264 files?

My operating system is OS X Version 10.6.1.

The error message I receive is:

The document “opening doc5 4x3.mov” could not be opened. The movie is not in a format that QuickTime Player understands.

You may need to install additional software to open this type of file.

The Tell me more button takes me here:

http://support.apple.com/kb/HT3775?viewlocale=en_US

What do I do?