I'm sorry your users are running into problems with the runtime install. Which version of the runtime are they trying to install? If this is occuring with the AIR 2.0.2 runtime, please be aware that the system requirements are for operating systems greater than OS 10.5 and Intel only CPU's.
What version of AIR are you installing?
Can you capture installation logging and post? Run installer again and see what the log captures.
For AIR 1.5 and earlier: http://kb2.adobe.com/cps/403/kb403123.html
For AIR 2.0 and higher: http://kb2.adobe.com/cps/839/cpsid_83989.html
There is one thing you might try. Sometimes the langauage lists on 10.4.11 gets corrupt which affects the installer. Try resetting the language list by adding/removing a language or reordering the list and then return to original setting.
-Go to System Preferences and select International.
-If you have more than one langauge on the list, reorder the bottom langauages by dragging one on top of other. Leave your top langauge alone (ENGLISH).
-Click Edit button and check or uncheck a language to add or remove.
I suggest to restart system and then try installer again.
Please note : AIR version 2.0 and greater are not supported on PPC.
I have a PowerPC running OS 10.4.11 - had been using Adobe Air until I had to clean install system & lost it. After much searching I finally found access to version 1.5.3 and have successfully installed it today. This is where I found it: http://mac.wareseeker.com/download/adobe-air-1.5.3.rar/c45a5e758 I hope the link works for you. I scrolled down to where it says Download location for Adobe AIR 1.5.3 & clicked on 'External download mirror' to download it. It took a while to install, but I have it. Good luck!