If you have an existing DLL project, then you need to include the Rhino SDK headers and link with the Rhino SDK import libraries. Often times it’s easier to just generate a new DLL project from scratch, and than add your functionality to it.
The open source version of openNURBS is intended for 3rd party application developers to use to add Rhino 3dm file reading and writing to their applications. It is not for Rhino plug-in use. Rhino already provide a much more full-featured version of openNURBS.
– Dale