What are you trying to do?
Almost all Mac OS X bundles are actually folders with a very specific layout of files/folders within. Rarely is a bundle just a file -- they do exist, but not often.
In most cases, the Bundle bit on the file is entirely irrelevant.
i want to set a folder as a bundle. so through the file system it will looks like a file. later a want to open that package with my application. the question is how to set the folder bundle attribute through cocoa.
You really don't need to set the bundle bit. As long as your folder has an extension (which it should) and your application is properly configured to open files/folders of that extension, it should sow up in the finder as if it were a file.
Example:
Go to ~/Desktop in Finder
In terminal, do:
cd ~/Desktop
mkdir aafoo
mv aafoo aafoo.rtfd
After (4), you'll see 'aafoo' show up in Finder as a folder.
AFter (5), 'aafoo' will change to be a TextEdit document. No attribute bits necessary.
OK -- fair enough. You really really want to set that bit.
Given that you have the command line to do it, I would suggest simply using NSTask. It will likely be easier than using an API to do so.