You can download Stacks from Yourhead’s website (here).
Install it by double-clicking on the file that you downloaded, after which RapidWeaver Classic will launch and ask you if you really want to instal Stacks.
After you restart RapidWeaver and open a Stacks project (or start a new Stacks project), you’ll notice that Stacks is now in trial mode (with limited functionality) and displays a banner urging you to buy it. Don’t buy it.
Instead, open lookup and enter the email address you used to buy Stacks originally. You’ll then get sent the registration code for Stacks, which unlocks the full version.