After getting Wario, go to the basement type place and go through one of the doors with no star on it. You will find a sign that says something about the two pillars in front of you. Ground Pound both of them and the water will sink. After that, go outside close to under the bridge to find one of those black brink type things -- make sure you have Wario. Ground pound it. Collect all the red coins to get the Star in that secret spot.