Help! Urine odor and stains

We are putting our house up for sale this week, and I have two little dogs and a pup (dog sitting the pup). The male has made a habit of marking one corner at the bottom of our basement steps. I have tried everything I could think of to remove the odor (steam cleaning repeatedly, febreeze for dogs, etc) to no avail. I actually have just removed the baseboard as it stinks and is soft and wet.
Any advice on how to get rid of this odor. I am keeping the dogs away from it, but I need to make it smell better QUICK!!!

its in the underpad....you will need to remove the underpad and clean the floor and back of the carpet.

for the stains.....try a mixture of 25% hydrogen peroxide and 75% water....test an area and let dry overnight.....if all is good the apply using a mist sprayer and let dry overnight,repeat if needed.