подключать библиотеку надо примерно так:

Код:
    internal class NativeMethods
    {
        [DllImportAttribute("owen_io.dll", EntryPoint = "OpenPort", CallingConvention = CallingConvention.StdCall)]
        public static extern int OpenPort(int n, int speed, int part, int bits, int stop, int vid);

        [DllImportAttribute("owen_io.dll", EntryPoint = "ClosePort", CallingConvention = CallingConvention.StdCall)]
        public static extern int ClosePort();

        [DllImportAttribute("owen_io.dll", EntryPoint = "ReadIEEE32", CallingConvention = CallingConvention.StdCall)]
        public static extern int ReadIEEE32(int adr, int adr_type, System.IntPtr command, ref float value, ref int time, int index);

        [DllImportAttribute("owen_io.dll", EntryPoint = "SetMaxRetriesGlobal", CallingConvention = CallingConvention.StdCall)]
        public static extern void SetMaxRetriesGlobal(int n);
    }