TheStu got this started in the wrong direction, and off we all go, wrong, wrong, wrong. Here is the full and correct way to play subtitles with QuickTime Player:
-- The movie file can be MKV or AVI.
-- Install Perian AND "srtimport.bundle" from this link:
http://www.applesolutions.com/bantha/srt.zip
Yes, it seems a belt-and-suspenders approach, but it works.
-- Be sure the System Preferences > Perian is set to "Load external subtitles".
-- QuickTime Player preferences should be set to "Show subtitles when available".
-- The file names MUST BE EXACTLY THE SAME before the extension, and NO DOUBLE EXTENSION. Example: "Splendor in the Grass.avi" and "Splendor in the Grass.srt". This will not work: "Splendor in the Grass.avi" and "Splendor In The Grass.srt".
Even with all this done correctly, occasionally it decides to fail, and when that happens, all AVI files fail to activate the subtitles. The only solution is to uninstall Perian, reinstall Perian, and restart.
With QuickTime Player X, the latest Perian (v. 1.2.3) is needed.