This is a passive tame creature. Meaning you cannot knock it out to tame it, it must be conscious and very much active to tame. By far the best method is to gather as many mejoberries as possible. After this, try to get to a non-hostile environment with the Diplo following behind. Then, stand under the breast of the animal (or right between its front legs) and just spam the Y button for Xbox (after placing the Mejoberries on the last slot of your hotbar) and just wait until it gives you the option again. The Diplo will not move unless disturbed by someone else. This Dino with a saddle is amazing for large and fast transportation of up to 10 passengers! With a decent amount of health too!
When taming to make it easier stand against a cliff or a good sized rock. I tamed one for my brother and it was a piece of cake with that technique. Also if you find that the place you find the diplodocus isn’t safe just get near it and lead it away. They won’t hurt you they just like to swat you around like they are playing a game of soccer.