Natures Miracle. You can get it at any pet store and it works great. It's an enzyme cleaner that neutralizes the ammonia in urine. it works great on clothes, carpets, etc. With a kennel full of sled dogs we use it to clean everything. Hope this helps.

Shoot the dog that did it, burn the pants and get a cat! lol

First off, stop letting your dog wear your pants! They don't have thumbs, so they can't make the zipper work.
Now that the pee is on the pants, try this:
Fill a spray bottle half full with water, then add 2 oz. of hydrogen peroxide, 2 oz. of vinegar, and 1/2 oz. of dish soap. Shake lightly to mix the solution while avoiding shaking so hard that you make the dish soap bubble up. Spray the peed-on area liberally and let it soak in for ten minutes. Next, sprinkle with baking soda and scrub it into the pee spot. Wash in cold water with your regular detergent.

Burn them