You are most likely using the wrong tool to do that job.
If all you want to do is re-assign which file gets opened with which application, then the Finder's Get Info window should be used for that purpose. You can do this on a per-file basis, or you can specify that the Finder should open all files with the given suffix or file type with a selected application.
If you want to edit resource forks, then the best place to do that is in Classic. Use ResEdit for that purpose.
Mac OS X applications now need to use bundles and Info.plist files (within the bundles) for telling the system what file types an application can open, with the file types being either the traditional file type and file creator data, or with the filename suffix. If you are still using applications written for Mac OS X 10.1 that often use the Macintosh file type and file creator data (eg. AppleWorks 6), it's better to get Classic to deal with the data files created by those older apps.
Why use Classic? Because Mac OS 9's Resource Manager is not the same as that used by Mac OS X. If you want to ensure that the changes you make to a document file is compatible to both Mac OS 9 and Mac OS X environments, making the changes in Classic via ResEdit will help ensure that your changes will actually work.
As you use applications designed for Mac OS X 10.2 or later, you'll find that hacking around with the resource forks of files will no longer be the means in which to correct the database that Launch Services uses to determine which application should open which file -- that role now belongs to the Finder by way of the Get Info window.
-- tonza
