Read the FAQ found in the stickies, if you haven't already. Here's a handy link: http://community.myfitnesspal.com/en/discussion/10098937/faq-syncing-logging-food-exercise-calorie-adjustments-activity-levels-accuracy/p1

You say you synced your Fitbit to your phone, but did you connect your MFP account to your Fitbit account? If not, that's what you need to do. If you have, then further troubleshooting may be necessary.