Then use Android's command line adb tool to install Runtime.apk. Apk files are Android package files.
On Flash Builder Burrito, the emulator Runtime.apk is at:
On a terminal, go to android sdk platform-tools folder
> ./adb -e install -r
> cd /Applications/android-sdk-mac_x86/platform-tools
> ./adb -e install -r "/Applications/Adobe Flash Builder Burrito/sdks/4.5.0/runtimes/air/android/Emulator/Runtime.apk"
After installation, you can go to emulator's settings-> applications -> android applications, you should see "Adobe AIR" as one of the application.