Entei can be found in any grass area. You have to be very lucky to find him somewhere. He will flee as soon as possible, so you could use a master ball on him.
I presume that you have released them, suiciune, entie and rioku (sorry i don't really remember how to spell them). Once you have released them, you can see that they are now seen on the pokemap as...
You can catch entei in stark mountain. First go catch heatran. Then go talk to buck in Resort area. He will say something about entei or volcano. Then go to stark mountain.And that's it. Good...