1. Does anyone know what to do with the ninja star?
Get the blue key and open the gate located in Star Zone (B). That leads to the Earth Zone. Use the ninja star in Earth Zone (B)
2. How can I find someone to help me push the very big ball in the circus tent?
You'll have to talk to Giant Gold who is located on the beach in the Sunny Zone (B)
3. Where can I find the swimsuit?
You get and Inner Tube, not a swimsuit. I think you get it after beating someone in a contest - don't quite remember.
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Now if someone can figure out how to catch the pink rocking horse located in Star Zone (A)?