I read an interesting article regarding Photoshop plugin creation here. Where the author says:

"In 2002 Adobe restricted access to the Photoshop Software Development Kit (SDK), which contains the specifications for Photoshop plugins, so it can be expected that these incompatibilities may increase. Developers of other image applications don't have access to it anymore, so they can't support newer plugin features. However, plugin developers who want to make their plugin compatible with a large variety of applications should still use the old SDK from Photoshop 3.05 or 4.0, which includes all important specifications."

Anyway, if using the SDK from version 4.0 would help, its located on the full Photoshop 4.0 CD, under "Adobe Technical Info:Developer Support:SDK:Photoshop 4.0 SDK". The file "Photoshop 4.0 SDK" looks like a document icon, but is actually the installer.

If you don't have the 4.0 CD, its here at the Garden (372+ MBs).