No idea what the dll is, but a dll usage example for VB would be:
Public Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Integer, ByVal dwShareMode As Integer, lpSecurityAttributes As IntPtr, ByVal dwCreationDisposition As Integer, ByVal dwFlagsAndAttributes As Integer, ByVal hTemplateFile As Integer) As Integer
The C# equiv is:
[DllImport("kernel32", EntryPoint = "CreateFileA", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
public static extern int CreateFile(string lpFileName, int dwDesiredAccess, int dwShareMode, IntPtr lpSecurityAttributes, int dwCreationDisposition, int dwFlagsAndAttributes, int hTemplateFile);
It doesn't really get much more complex than that, so providing you know the functions in the dll you need to call it should be pretty easy - if you don't the only easy way to find out is through the documentation.