Neoprene cement is all you need. (You're talking about the smelly black stuff, right?)
Hopefully you bought the can with the built-in brush, and not the tube. The tube is better for getting into smaller wetsuit rips. But for this job, you'll need to brush the cement over the full contact area.
Pay attention to the edges: you want to make sure it's well bonded all the way around. On the suit's thigh, you should mark out the pocket location, and "over glue" by 1/4 inch or so, to make sure the entire edge of your pocket is well bonded all the way around.
You'll want to do two coats of cement on each side, pocket and suit. The first coat should saturate and bond to the fabric. You'll notice it soaking in as you spread it. Let that coat dry. Then do a second coat on each side. This coat shouldn't soak in: it should just create a nice bonding layer on top of the first coat.
When the second coat dries, press your pocket onto the suit slowly and carefully. You should get an instant "contact bond" that will last a good long time.