can I sync with my Fitbit Flex

Connect your accounts at http://www.myfitnesspal.com/fitbit

Set your goal to .5 lb. per week for every 25 lbs. you're overweight: http://www.myfitnesspal.com/account/change_goals_guided

Enable negative calorie adjustments: http://www.myfitnesspal.com/account/diary_settings

In the MFP app, go to More > Steps and choose Fitbit.

Ignore your Fitbit calorie goal and follow MFP's, eating back your adjustments. No need to log any step-based activity—your Fitbit is tracking it for you. Log non-step exercise (like swimming or biking) either in Fitbit or in MFP—never both. Exercise logged in MFP overwrites your Fitbit burn during that time.

You can learn more in the Fitbit Users group: http://community.myfitnesspal.com/en/group/1290-fitbit-users