i say, eat when you are hungry, but, make sure it is hungry - not boredom hungry, and when poss, choose healthy options, which can be hard when you have m/s and the only thing that you feel like eating and that stays down is junky.
I read somewhere that you only need to eat an extra 200cal a day, or something like that, equilivent to an extra peice of fruit. So, if that is correct, its not a heck of a lot.
In your first trim, putting on anywhere up to 2.5 - to 3kg is pretty acceptable i think is the recommended weight gain.
Having read some of your other posts where you mention you have had varing weight issues i say try not to worry to much about what you weigh, just keep active, and eat as healthy as poss. Chuck in veges with those carb meals to help fill you up.
The best thing i did - from the advice i got on here, is to stop being paranoid about my weight. I do weigh myself everyday, but, i dont record it daily - (one step at a time!) as i really have realised that it changes so much each day, depending on fluids etc.