I run into the same problem with you.
Have you found the way to solve this, if so could you share the way to fix the problem.
Since this post is very old, you may need to post a new thread in the LabWindows/CVI forum to get a faster answer. I would recommend posting there rather than the LabVIEW forum, if your DLL reference is null in the LabWindows/CVI code as in the original question. The users there will be more familiar with troubleshooting dynamically loading a DLL in C.
It would also be helpful to see the code that loads your DLL, and details about how the DLLs are installed on your system.