woah, okay. tried out what Chrono79 Suggested. got error and now I am confused. Any hints why could that be happening?
Test Failed
Expected string length 217 but was 230. Strings differ at index 147.
Expected: "...9000000000000 + 70000000000 + 8000000000 + 50000000 + 3000..."
But was: "...9000000000000 + 70000000000 + -4000000000 + -600000000 + 5..."
------------------------------------------------------------------^

Also Tried this:
Assert.That(Kata.ExpandedForm(60000000000000009), Is.EqualTo("60000000000000000 + 9"));

Test Failed
Expected string length 21 but was 65. Strings differ at index 20.
Expected: "...0000000000000 + 9"
But was: "...0000000000000 + 40000000 + -4000000 + -20000 + 200 + -40 + -5"
------------------------------^

please be more specific with your problem. you should print only to test it out. If you want to pass this kata you have to return a value like in most katas