The wires can run exposed in your wall. They make two types of HDMI cables. One for in-wall runs and the other exposed. I've found them both to work well in-wall. The only concern I would have is how far is your run to your plasma?

If you are asking "how" to hide the wire within an existing wall...

You need to remove the baseboard or crown molding so you can run it down and along the floor or up and around the ceiling to get to the receiver.

I did this with speaker wire. Luckily I have about 18" space above my ceiling so it was really easy to fish across the room. But we had a header in the corner of ceiling so we had to drill at an angle from ceiling to wall to run the wire down to the recepticle on wall. But we were able to cover the little bit of wire with crown molding.

They sell hdmi plates and connectors so you can make a professional installation.

Do some research on the length of your cable, there seems to be lots of opinions about how far you can or cannot go from receiver to tv.

Good luck.

Oh, another trick. I had to hide wire but was unable to get between the studs in wall. So I dug out a trench in the corner or room. Was able to shove the wire in there and then covered it with caulking. Because I took my time and only made a tiny trench, you cannot see that it was even done. I could have put more joint compound over it to make it blend even more it I had to, but the trench was so tiny it blends right in.