This has to be unlocked in free play.
The amber brick to unlock the Velociraptor is found in the fourth level from Jurassic Park, 'Restoring Power'.
Shortly after entering the underground tunnel you come to a fork. Turning left continues the level with Ellie and Muldoon, turning right leads you to the amber brick with can be found after Ellie interacts with some dung.
Once freeplay is unlocked, set both characters to the new Raptor. The baby Raptor character model works ok for this achievement.