Currently .NET is not a supported platform for that driver. I know some people have had success getting things to work, but it will take some know-how on your part. Here are some comments from another .NET user with the NI-435x driver. This customer was using C#.
1) The NI 435x driver must be installed
2) LabWindows/CVI support must be installed (under DAQ traditional drivers)
3) Each DLL method that is to be called must be declared using the DllImport attribute and both the extern and static modifiers.
For example, the following is the declaration for the NI435X_Getting_Started_No_Acc method:
[DllImport("ni435x_32.dll")]
public static extern int NI435X_Getting_Started_No_Acc(string resourceName, string channelString, short
mode, double lowLimit, double highLimit, int numberOfScans, double[] scansData);
4) The following system namespace must be included to add support for the DLL Import and external method calls:
using System.Runtime.InteropServices;
5) The ni435x.dll needs to be linked as a reference for the project. This will also allow access to the constants defined within the DLL.
For example, the NI435X_VOLTAGE constant is used in the following:
short mode = (short) ni435x.DLLConstants.NI435X_VOLTAGE;
6) More information on using importing DLLs and external method calls can be found in Microsoft C# Language Specification - section 10.5.7 External methods, and in Microsoft C# Programmer's Reference - Platform Invoke Tutorial.