All the DLC is included with the patches, including the preorder DLC.
You obviously don't have a game patched up to date, because 1.2 is the latest patch, and that patch removes the DLC button from the launcher.
You should download patch 1.1 (because I suspect you're assuming that it automatically patched up to date when it was patching out the DRM) from where the download links are in "my account," if you scroll down to the bottom of the list.
Your game should then patch to 1.2 automatically if you're connected to the internet, or you can download the patch manually from
here.
EDIT: I'm including a screenshot of where you can get the GOG-specific 1.1 patch.