Refer to the official documentation by Apple regarding generating the icns icon here

You need to have matched the requirements or the Application Loader will fail.

• icon_16x16.png
• icon_16x16@2x.png
• icon_32x32.png
• icon_32x32@2x.png
• icon_128x128.png
• icon_128x128@2x.png
• icon_256x256.png
• icon_256x256@2x.png
• icon_512x512.png
• icon_512x512@2x.png

1. Create an iconset $ mkdir [name].iconset
2. Add all the PNG's to the iconset folder
3. Run the icon util $ iconutil -c icns <iconset filename>
4. Replace the app.icns and the document.icns file with the generated icns inside your.app/Contents/Resources