Load into a single player game. Upon loading press the menu button to pause the game. In the console line at the top of the screen type in "summon rex_character_bp_c" without the quotes and press admin command. Quickly unpause and then pause again the game so that you can see the name of the trex with it's level and then in the box type "forcetame" and press the admin command button. This will tame the dino and you will have your achievement after you hop on. Force tamed dinos do not require saddles.
You must aim your cursor at the dino in order for force tame to work. So make sure you unpause and then pause again so you are pointing at it after summons.
If you are having problems with this you may want to type "god" without quotes into the box and press command to start god mode so you don't get eaten alive.