Boy have I been there before! I used Woolite OXY DEEP spot and stain carpet cleaner. It's in a blue spray bottle. I spray it on and then take a hard bristled carpet brush and scrub it, then blot it. It did really great for me. My son went through a phase of taking off his stinky pull ups and smearing it all in the carpet.
Good luck and I hope this helps!!
P.S. I recently purchased a Spot Bot (made by Bissel I think) and it is wonderful. Even my hubby said yesterday (after using it to shampoo the carpet and seats in his truck) that it is worth every penny we paid for it. But the Woolite and brush works great too and it really takes the smell out too. Leaves it smelling pretty fresh actually.