Big question is: Are you on a PPC mac or Intel?
(Edit)
PPC or Intel is irrelevant from what I've found.
http://www.oldapps.com/mac/adobe_air_for_mac.php?system=mac_os_x_10.5_ leopard_powerpc
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.
