at . (Int32 , Int32 , Byte[] , Int32 )
   at .C_Login(Int32 , Int32 , String )
   at Cryptware.NCryptoki.Session.Login(Int32 userType, String pin)
   at Cryptware.NDigitSign.DigitalSigner.Login(String pin)
   at FullTestNDigitSign.Form1.button4_Click(Object sender, EventArgs e) in d:\Users\Tito\Documents\Projects\TKS\Digital\FullTestNDigitSign\Form1.cs:line 45
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at FullTestNDigitSign.Program.Main() in d:\Users\Tito\Documents\Projects\TKS\Digital\FullTestNDigitSign\Program.cs:line 17
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Not exactly sure about your question, but there is no pin being passed. There is also no code that sets the encoding. The certificates are pre-configured as a mean of authentication between the client and the server.

Sorry, it is .NET string hardcoded into the sourcecode. It contains @ sign, English letters, and number

I tried to input other PIN number such as 1234 or even left blank, but got the same error. 

Try:

digitalSigner = new DigitalSigner(@"C:\\Progra~1\\LunaSA\\cryptoki.dll");