If you are primarily trying to just call a C library which interfaces with your hardware, then I definately suggest going with the DLL path rather than CIN. It is much easier to build and maintain.
I really good starting point in terms of designing your C API for LabVIEW is to consider what kind of data you will be passing back and forth. I recommend downloading this example and looking over it. Let me know if you have any questions.
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3E78F56A4E034080020E74861
Note that it isn't a CIN, even though it does make use of the extcode.h and certain calls into LabVIEW. It is just one DLL calling into another.