For whatever reason, C18 is only available through the spoilers option right now.
Cockatrice > Settings > Deck Editor
Check "Update Spoilers Automatically," then click Update Spoilers. Finally, make sure C18 is enabled under Card Database > Manage sets.
If you don't see the Update Spoilers Automatically option, go to Help and update. If it won't update, uninstall (you can keep your settings) and get the latest release.
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