I group of us have started playing a campaign game. On our second mission when the doors opened, I deployed some figures in the middle of a round. Do the new deployed figure (imperials) get to activate or do they have to wait till start of new round?

Hi wolfhelm, the newly deployed figures would activate immediately following the end of the current Rebel players turn in the order you choose. You would alternate back and forth unless there were no Rebel players left to take their turn.

All deployment cards enter the game ready , unless specified otherwise by the mission event, so they will get to activate during the same round.

Rebel Lesson #1: Never open a door on the 3rd or 4th rebel activation.

.. and not as the second action if you can help it. (Unless you're Mak with Covert, Disengage or Expertise.)

Second action is acceptable in most circumstances, since time is usually a factor, but first is ideal.