Your plugins will probably need recompiling against the new headers and the new ac3d.lib (which is included in the Windows release)
The old SDK is on the registered users web site at
www.ac3d.org/private.
This has not been updated for 4.0 yet, but a lot of the old SDK still applies.
If you would like a copy of the latest header files, email me and I'll send them.
Until the new SDK is available, please post any questions with regard to development here.
Andy