After unlocking the 3rd island you will be able to send dig teams to other areas.
In the middle of North America you will unlock sites for Brachiosaurus and Diplodocus at the same time.
Simply send expedition teams to retrieve Brachiosaurus fossils and use the fossil lab to get your genome above 50%.
Once at 50% you can use the Hammond labs to create a Brachiosaurus.
I found the achievement wouldn't unlock but did so after forcing the game off and restarting by going to the home page and pressing the start button whilst on the Jurassic Park button. This opens the menu, the bottom option I believe should be to quit the game, press this and then after it has turned the game off simply restart and the progress towards the achievement should've caught up.