If you've lost your Aether license key, or have reinstalled it and see a message that Aether is running in demo mode, this page is for you. See the appropriate section below depending on whether you bought Aether directly from the Aether website, or from the Mac App Store.
When you purchase Aether, your license key is emailed to you. If you still have the email you received, you can simply enter the license key it contains after reinstalling Aether or installing it on a new machine. If you can't find your license key or the email you received, please contact Aether support. I do my best to respond to license recovery requests as quickly as possible (usually the same day). Make sure you include the name you used when purchasing the license so I can look it up easily.
App Store Version
If you purchased Aether on the Mac App Store, you will not receive a license key, nor do you need one. Apps purchased on the Mac App Store are licensed by the app store itself. If you purchased on the app store, but you're seeing the notice that Aether is in demo mode, it means that you have installed a copy of Aether downloaded directly from the Aether website. To fix the issue, you should delete that copy of Aether by dragging it to the trash, then reinstall it using the Mac App Store. Follow the instructions here to install it.