after spending a couple of days with this matter I have to look for some help. Reading and applying all hints (including those from the forum) I still get these messages:

bb:mtw ok$ sencha package build -p packager.json
[INFO] The application was successfully packaged
Failed to execute system command while signing application with error 256
Failed to package application
[ERROR] /Users/xx/Sites/deploy//appname.app: cannot find code object on disk
Despite the error message an app-file is generated. I have also moved that app-file to iTunes but it wont synchronize with a device (iPhone).

I guess I am missing something.

Also when looking at the app-file, it looks like it contains the whole contents from the source folder, which is much more than is needed to run the app (e.g. css-debug and touch-all-debug). But that is maybe something for later optimization.