How to weight pedals?
The pedals you have are known as mountain bike clipless pedals. They have a platform side where you can use normal sneakers/shoes to ride, and the other side with the "hump" is the clipless side. There are special shoes with cleats on the bottom that attach to the pedals,and they're there to increase your pedaling capacity by allowing you to pull up on them as well as push down. As for weighting the pedals I wouldn't know how as I don't have the pedals in my hand, but I'm sure that if you were to go to the hardware store with the pedals, you may be able to figure something out. I'm sure that duct taping a weight to the "hump" side would solve your problem, but keep in mind that you are adding mass to the pedals, and it may make contact with the ground (aka pedal strike) during a sharper turn.

My suggestion is to go to your local bicycle shop and buy some platforms pedals so that you don't have to go through the trouble of modding your pedals, or have the issue of extra weight, or pedal strike. And considering that cheaper platform pedals are around 15-30 dollars, I think it's a worthwhile investment as they'll be functional on both sides, without the trouble of fabricating a weight attachment.