Managed dll import complaining about string marshalling

Hi folks, I am using a Managed dll to use Measuring Computing's DAQ, a piece of hardware that lets me do fancy analogue I/O. I'm able to import the dll, instantiate the class, and even access fields, but when I try to call a method, I get: NotImplementedException: string marshalling conversion 34 not implemented. I guess my first question is: Is it OK to use a managed dll that calls native dlls? I've run into a problem in the past where I couldn't have one dll access another, but they were both c++. The second question is: could I get this exception by simply not having the right header or required native dll accessible? Any guidance would be much appreciated! Alex