How do I use the STIX Font in Mac OS X 10.7
These fonts should get used automatically by standard marked-up text that calls for them.  Check this test site with FireFox 4 and up:

https://eyeasme.com/Joe/MathML/MathML_browser_test

Info on documentation of the fonts is at

http://voxel.dl.sourceforge.net/project/stixfonts/DocumentationOnly/STIXv1.0.0.pdf

Info on using STIX fonts with LaTeX is at

http://www.charlietanksley.net/philtex/the-unicode-math-package-for-xelatex-and-the-stix-fonts/