There used to be a Cakewalk DevXchange with information on Cakewalk SDKs, notably the control surface SDK.
 
DevXchange is no more and one is invited to consult calkewalk blog for "the latest developer notes, sdk's and format information".
 
I've searched for SDK on the blog, but there's very little, and nothing on control surface SDK.
 
Where can I download it ?
 

Try looking on Github.
 
Its been put in the public domain for further development.

https://github.com/Cakewalk/

Thanks!
 

It's worth your while having a chat with azslow3 as he has done a lot of work with a control surface plugin that has proven very successful.