In my guest room of my home which is 5 years old, I smell a new odor that appears to come from the wall. The odor is very strong and smells like a dead animal odor. I have not heard any mice in the attic and cannot figure out what the odor is. My questions is, how do you get rid of such an odor in a wall?

I would check electrical outlet boxes, heating vents, anything you can "open" and look into. Do you have a crawl space or attic that you can check?
If it's a dead animal, chances are the smell will go away on it's own. In the mean time, you might try some commercial odor cover-up products.

Exact same problem - however, I had some holes cut but could see nothing. Smells like a dead animal (not mold). Are there chemicals that can be injected into the walls to eliminate (vs. coverup) the smell??