I don't understand what's the hint in this problem! I read it and I think this code must be enough to solve that problem in c#:
using System;

namespace ConsoleApplication1
{
    class Program1243
    {
        static void Main()
        {
            ulong n = Convert.ToUInt64(Console.ReadLine());
            Console.WriteLine(n%7);

        }
    }
}

But I get WA#5. And I don't understand! Can someone help me? . . .

10^50 does not fit into UInt64. BigInteger integer does.
However you could try to divide string by 7, or use some more sophiscated math (O(N), no divisions at all).