You unlock the timeline by picking up black boxes and by playing. I got half of them while playing though all the skirmish maps and with a few black boxes. I got the achievement for unlocking all of them when I picked up my last box, having played the whole campaign and every skirmish map.
I bet you can find a guide to where the black boxes are. If you don't wanna do that you can always get a Hornet and fly around until you find it. It's usually kinda hidden so you might have to look a while.