You need to add executable privileges for icns.sh after the first time you run the command. If you changed your project name, you need to remove icns.sh and rerun the command.

Generate Iconset.jsf

/***********************************
Adobe Fireworks Command File

Auto export iconset with different
resolutions for icns generating.

Install into ~/Library/Application 
Support/Adobe/Fireworks CS6/Commands

************************************
The MIT License (MIT)

Copyright (c) 2013 RixTox

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
***********************************/

var dom = fw.getDocumentDOM()
  , dir = Files.getDirectory(dom.filePathForSave) + "/"
	, fullname = Files.getFilename(dom.filePathForSave)
	, extension = Files.getExtension(dom.filePathForSave)
	, filename = fullname.substring(0
		, fullname.lastIndexOf(extension))
	, outdir = dir + filename + ".iconset/"
	, shdir = dir + "icns.sh"
	, sizes = [16,32,128,256,512]
	, scales = [1,2];

if(!Files.isDirectory(outdir))
	Files.deleteFile(outdir);
if(!Files.exists(outdir))
	Files.createDirectory(outdir);
if(!Files.exists(shdir)) {
	Files.deleteFileIfExisting(shdir);
	Files.createFile(shdir, "TEXT", "FWMX");
	var batch_file = Files.open(shdir, true);
	batch_file.write("iconutil -c icns \"./" + filename
		+ ".iconset\" \"./" + filename + ".icns\"");
	batch_file.close();
}
	
for(var i in sizes) {
	for( var j in scales) {
		fw.exportDocumentAs(null, outdir + "icon_"
			+ sizes[i] + "x" + sizes[i] + (scales[j]==2?"@2x":"")
			+ extension, {
				exportFormat: "PNG"
				, colorMode: "32 bit"
				, jpegQuality: 100
				, paletteMode: "exact"
				, paletteTransparencyType: "index alpha"
				, useScale: false
				, applyScale: true
				, xSize: (sizes[i] * scales[j])
				, ySize: (sizes[i] * scales[j])
		});
	}
}

alert("Finished.\nPlease add executable privileges to icns.sh file.")