You can download the SFZ and the samples from my website:
http://lilypondtool.organum.hu/garritan-aria-sfz-tutorial-my-whistle.zip
You can watch the video here:
Some useful links:
- The SFZ speification: http://www.cakewalk.com/DevXchange/article.aspx?aid=108
- An other SFZ tutorial (though not Aria-specific): http://www.soundonsound.com/sos/apr08/articles/sonartech_0408.htm
- SFZ forum on Plogue's (Aria's developer) site: http://www.plogue.com/phpBB2/viewforum.php?f=14