push ebp
mov ebp, esp
mov eax, [ebp+0x0C]
mov ecx, [ebp+0x8]
add eax, ecx
pop ebp
ret 0x8
    

push rbp
mov rbp, rsp
xor rax, rax
xor rcx, rcx
mov eax, dword [rbp+0x0C]
mov ecx, dword [rbp+0x10]
add rax, rcx
pop rbp
ret 0x8

    

Your offset into that stack are wrong. Each 64-bit push is 8 bytes.

push rbp
mov rbp, rsp
xor rax, rax
xor rcx, rcx
mov rax, qword[rbp+0x10]
mov rcx, qword[rbp+0x18]
add rax, rcx
pop rbp
ret 0x10
    

  private delegate int DelAddNative(int a, int b);


  private static Int32 AddNative(int a, int b)
        {

            Int32 OutRes = 0;
// Allocate a Handle
                GCHandle PinnedArray = GCHandle.Alloc(x64Addition, GCHandleType.Pinned);

                // Get handle for shellcode
                // Get address of Object
                IntPtr ShellcodePointer = PinnedArray.AddrOfPinnedObject();

                // Convert function-pointer to Delegate
                DelAddNative AddDelegate = (DelAddNative)Marshal.GetDelegateForFunctionPointer(ShellcodePointer, typeof(DelAddNative));

                uint flOldProtect;

                // Make shellcode executable
                VirtualProtect(ShellcodePointer, (UIntPtr)x64Addition.Length, PAGE_EXECUTE_READWRITE, out flOldProtect);

                // Execute shellcode
                OutRes = AddDelegate(a, b);

                // Restore old flag
                VirtualProtect(ShellcodePointer, (UIntPtr)x64Addition.Length, flOldProtect, out flOldProtect);

                // Release handle
                PinnedArray.Free();

   return OutRes;
}
    

x64Addition:
  mov rax,rcx  ;parameter 1
  add rax,rdx  ;parameter 2
  ret    

x64Addition:
  lea rax,[rcx+rdx]
  ret