This creature does not need to be knocked out, it is a passive tame which means that you have to passively feed it by hand. This can be done by putting the food or kibble in your last hot-bar slot and pressing "E" (or "Y" on Xbox One) to feed it. Make sure to keep track of when it needs to be fed or else taming efficiency will slowly go down. The type of food you choose can also affect efficiency with kibble giving the highest and other berries giving the lowest. However, kibble can be harder to get in early stages.
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.