Well, I've caught six in the last month. The key is to find its hole. Once you find its den, its just a matter of time. I have caught them using three different techniques.
First and easiest if they are hungry is to cut a section of schedule 80 PVC and drill some holes in it. Tie a thread to one end and block the other end with a piece of pantyhose and a rubberband. Bait the trap with some raw seafood. Ive used small chunks of mussels, oysters, ocean perch, and shrimp from the local grocery store. Place the bait 3-4 inches inside the trap using a toothpick to skewer it through one of your drilled holes. Put it in the tank very near its hole (within 1 inch), and hold onto the thread. If he is hungry, he will probably come looking right away. If not, try again later. You must stay and watch, as they go in and quickly come back out. Without the toothpick, they will just steal your bait very quickly. This happened at first, and the shrimp was in and out with the bait in about 4 sec. As soon as he goes in after the bait, yank him out quickly. I caught 4 mantis this way. BTW, they are usually very interested at first, but must check out all of the tube before they venture in. This may take up to 1/2 hr of them testing the tube before they go in. So crack a beer and sit back and go fishing for some shrimp. It is actually pretty fun.
Another way is to start a siphon at the hole entrance with atleast a 1/2 diameter tubing. Then try poking from the other opening as most will find a lair with a back door. He will usually come shooting out after a few pokes from the other end. I caught one with this method that wouldn't go into my traps, but would allow me to hand feed it from a tweezers. A smart one, but I outsmarted him.
Lastly, if the rock is small, and you can easily access it, pull it out and poke in the hole over a bucket of saltwater, and it will most likely come shooting out the back door. I caught one this way too.
Good luck!
Andy