Harold,
It is very possible that LabVIEW doesn't know the data types in the header file. Generally, if your functions contain complex data types, we recommend calling the dll using a Call Library Function Node.
There is a very helpful example in the Labview Example finder that explains how to handle several different data types to be used with the Call Library Function Node. The example is named Call DLL.vi. Refer to this example when dealing with complex data types such as arrays or structs. To Find this example go to the LabVIEW Example Finder (Help»Find Examples) and select the Directory Structure radio button. In the directories list, select dll and then data passing then Call Native Code.llb. The Call DLL.vi example is under that directory.
Regards,
Leah
