Now Activities is the name of the Fitbit tab that shows daily graph of steps and calories.

So I'm assuming if MFP shows steps from Fitbit (do they match), then steps are showing up on Fitbit too.

But there are no Activities that are listed separately, Fitbit has no idea a walk you did was for exercise and not just daily step activity.

Or did you log an exercise on MFP?

Or do you mean rather MFP Exercise tap is not showing any logged exercise?
Same thing, that does not come from Fitbit to MFP, merely steps taken, and your total daily burn.
MFP then compares that total daily burn to what it estimated you were going to burn.
If they match, no adjustments.
If Fitbit said you burned more, whether because of exercise or just increased daily activity, then MFP gives you positive adjustment under exercise, so MFP also shows you as burning more that day.

But you must have positive and negative adjustments enabled on MFP - Settings - Diary Settings - Negative adjustments.

Sorry if I hit too many points, but you aren't clear on where you think there is an issues, and what you did or did not do.
Did you exercise today?
If so where did you log it?
Where are you expecting it to show that it is not?