I've tamed one on official. You have to do a certain number of unique missions, can't be the same one over and over. Once you've done that then you have to go to the middle of the map between rings to get mutagel. If you don't see mutagel in between the rings you have to wait until the next hyperspace jump. Hyperspace jumps change the landscape and material spawns in the no man's land area. Once you have mutagel and the correct number of missions go up to a Stryder and feed it the gel from the last slot in your inventory. Once you've done that, a body part will start sparking. It will be either the head, shoulders, leg joints, feet or chest. Go up to that body part and hit the Y button again. You will then have to play a mini game 3 times where you stop the scrolling blocks on the center line with RT. Then another body part will start sparking and after it has walked a certain distance away just prince and repeat. You only need the mutagel once.
To tame them, after completing the proper missions and getting mutagel, stand on their back and feed the mutagel to them. A peice of its body will spark and you will have to do a mini game where you use the attack button when the boxes are in the middle of the line. Messing up puts taming back to 0, but they will not need more mutagel.