Have a look at
http://developer.avid.com/
You can get the SDK from there and browse the sources. But you cannot do much with it before you got signed in with AVID. The instructions can be found on that page too.
There is no signing required anymore. I recently asked if I needed to sign anything before starting to support AAX and I got a friendly message back: not needed.
You do have to create an account before you can access the SDK and tools.
Of course it can’t hurt to ask them yourselves as well if you want to be 100% sure / covered.
Thank you
Got authorized and they provided access to the SDK.
Ho do you install it and use it with Juce?
I have not been able to find a guide.
I built the AAX library, and set the Global Path to the SDK folder.
Now the project builds and the plugin file is much bigger than 4K.