You should answer "Yes" to the question "did you forfeit or carry over any funds" when you first enter your W-2 with the FSA in box 10. You will not be taxed for the unused portion if your employers allow you to carry over the unspent amount into February next year. If you missed that question, try deleting the W-2 and entering it again.
Please see the post by Opus 17 for more information.