The AC3 encoding has shifted to Compressor now, so any reference to A Pack should be ignored.
Follow Floh's direction to find the AC-3, but beware that it defaults the "Dialog Normalization" to -27dBFS. If you've got your audio mixed how you want to, you can set this to -31dBFS. This stops any attenuation happening to your audio file. Otherwise your audio might sound quieter than you expect.
There are obviously good reasons to use the Dialog Normalization - but that's a discussion best left for you and the manual :o)
HTH
Ben
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