The Call Library Node, which is how you would normally make such a call, isn't going to be very straightforward because it doesn't understand OLESTR strings. I'm not sure exactly how your program is working, but you might want to consider using the .NET System.Guid class. This has automatic support in LabVIEW for converting the data types back and forth. You can pass in a array of bytes and get a string version out, and vice versa.
This solution does require at least LV 7 and installing the .NET framework, but it is a better integrated solution than the Call Library Node in this case.
