SPELL_AURA_APPLIED is not an event, per se. you need to register COMBAT_LOG_UNFILTERED as your event, and then parse the combat log event's arguments to figure out what type of combat log event occurred. this is how you detect SPELL_AURA_APPLIED.
and, fwiw, this particular combat log event has had some problems in the recent past with not actually registering for proc-oriented auras. not sure it's been fixed. most addons moved to a manual aura check using UnitAura() at strategic times (or ever x seconds).
http://www.wowwiki.com/API_COMBAT_LOG_EVENT