Big question is: Are you on a PPC mac or Intel?
PPC or Intel is irrelevant from what I've found.
Mac OS 10.5 is the minumin to run AIR
Adobe has Air 1.5.3 http://helpx.adobe.com/air/kb/archived-air-sdk-version.html But there is no support for PPC or 10.4 mentioned.
Thank you for the response. The Mac has Intel Core 2 Duo.