You need to apply to Digidesign to get the SDK, check
www.digidevelopers.com.
I tried a few years ago, and had to jump through loads of hoops and got nowhere. I've been told the process has improved since then.
Digi are meant to give you more support in a trade of for that, they also used to put some pressure on not to develop VST plugins - they wanted to keep you RTAS/TDM only - but again I think this may have changed.
Good luck!
Pete