Here's how I make app icons for Mac App Store. It works for me and haven't been questioned by the reviewers yet.
1. Create a 1024x1024 icon using Pixelmator/Photoshop/Acorn/your other favorite application, save it as png (with a transparent background). Do NOT use Icon Composer, it is not recommended by Apple, as you cannot create Retina (1024x1024) icons with the current version.
2. Download https://github.com/audiocommander/Iconset-Generator
3. Open the Generate Iconset.workflow in Automator, select the png file in Finder, click Run in Automator.
4. The workflow creates a folder on the Desktop, containing the full set of icon sizes (from the original png file).
5. Open Terminal and type:
iconutil -c icns ~/Desktop/myIcon.iconset
It generates the correct icns file, ready to be dragged to RB Package Maker Studio (or whatever tool you use) to create a codesigned bundle for MAS. The 512x512@2x icon size is in fact the original 1024x1024 icon image from your png file.