Robert,
In order for LabVIEW to be capable of calling compiled functions of C from a running VI, the code must be loaded and unloaded into memory at the same time as the VI code itself. LabVIEW uses object code in the form of a .lsb file which is generated by the C compiler from the C source code using the ntlvsbm.mak file. This ntlvsbm.mak file is included in the CINtools directory in the LabVIEW folder. Do you have this file in this folder?
I've attached this file from my LabVIEW 7 folder.