can I eat whenever?

I sometimes find myself having a handful of nuts or something, late in the evening, to boost my calories for the day. You can eat what you like, when you like within your calorie allocation. You'll just find that some foods are more satiating than others. Others taste good, but don't have much in the way of nutrients that are of benefit.

Just try to eat as close as possible to the number of calories that MFP has allocated for you. You shouldn't be aiming to eat less than that each day*. Some days you may have a bit more and some days you may eat a bit less. Not sure that it's on the desktop version but if you use the app on your phone, at the bottom of your diary is a Nutrition option that allows you to see your average calories over 7 days. A lot of people, myself included, tend to aim more for the weekly average to be as close to target as possible. This allows for days when I've exercised a lot and 'earned' more calories but not eaten them all, but also allows for a nice brunch or a glass of wine with friends over a meal at the weekend etc.

*every so often, as your weight decreases, adjust your stats in MFP and you'll find that it also adjusts your calories allocated.

Eat whenever and whatever you like.

For fat loss, stay around your calorie goal, plus exercise calories.

If you are above the minimum cals for the day, but still have calories remaining and are not hungry, you can save them for later in the week.
You can work in weekly calories as welll as daily.

For health, aim to meet your nutritional goals, most days.
Treat the MFP Protien and fat goals as minimums, let carbs fall where they may.

Cheers, h.