I just got off the phone with Paychex. Despite what http://www.paychex.com/~jlconway said in the April 19th email, I was supposed to have signed up for the Flex Spending Account between September 1 and September 30 in order to enroll for 2012, not October 1. I.e., it is the beginning of the month that is exactly 6 months from the hire date, not the beginning of the next month following the 6 months-from-hire-date. So, I missed the window.