that part is easy to miss if you try and rush the problem. you are investing 100 a year from now (that is, at t = 1) an not right-away. 12 years from now, 11 years will have passed since you invested 100 dollars, which means N should be 11 as you pointed out.
the magician is asking what value would you have to enter in I/Y so that, after entering N and PV you can find the future value using FV. it is worth noting that there is no dedicated button for continuous compound rate, though you could use iconv, set 8% as the nominal rate and an arbitrarily high value for the C/Y (99999 should be good enough), then asking the calculator for the effective rate. the EFF rate given by iconv is slightly below the true effective rate, but should be very close, and certainly close enough to the true effective rate for solving the problem.
having said that, i see no real reason to use the TVM functionality for this problem when it is cumbersome (you have to use iconv to even get the interest rate you need) and prone to error (if your P/Y was set to a number other than 1 as a result of solving another problem and forgot to change it back to 1, you would get a wrong answer as a result, even if your other inputs were correct)