https://web.archive.org/web/20160715111807/http://www.housekeepinghere.com/weekly/baking-soda-carpets/ says that I should keep the carpet dry, while http://www.repair-home.com/flooring/carpet/carpet-cleaning-solutions.html says that I should spray the area with water.
Baking soda needs to be in contact with the odors to work. If the baking soda is wet, it will not really work at all because the water will be blocking the odors from getting near the surface of the baking soda. I also suspect it would be a real mess to clean up.
In the past I've tried baking soda on carpets (dry!) and found it to be a waste of time. A professional steam cleaning is probably the most effective solution.
I use baking soda all over my rugs, carpets, and the dog's bed a couple times a year. I think it works great. This is just normal house/dog smell I'm trying to get rid of, not smoke or animal excrement thankfully. I wouldn't wet it bc like a previous commenter mentioned it would become clumpy and not as easy to vacuum up.