Hi i'll try to help. You should be able to drag the profession icons onto one of the mice my holding your mouse button down, and then let go the mouse button. Then they will take up the profesdion, does that work for you? Keep in mind though that you need to build a forest/shed first so they can collect apples, otherwise you might not have enough resources to assign a profession. When you hover over an icon, you will see it's resource cost.
I hope this helps a bit?
All the best!