Okay, I'm having trouble recalling exactly what the level looks like, but I beleive it is the level with the rising and falling Yellow ground, correct? If this is true, then simply play through the level (without Yoshi) until you reach the third section. In this section, the ground rises but doesn't fall. What you want to do is escape the rising ground, but don't jump off of it! Simply get as far right as you can to not get crushed, then jump up and over the ceiling and run left to the hidden keyhole. That will take you to the Valley Fortress, which will take you to the back door!