There should be directions in the email you received after completing the purchase. In summary, you'll need to get Stardock Central from the downloads page, install it, log in, and (possibly, it may link automatically) go to the tools menu and register the serial number from the purchase email. Then back on the main screen of SDC, go to the games section, click the + next to GalCiv2 to expand it, and you'll see DA. Select it and click install.
Do note that you need to have the regular GalCiv2 already registered on your account in order to be able to even download DA.
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