NEVER access files in the iPhoto library with any software other then iPhoto. OK yes there are safe ways to do it but if you are asking this question here, you are not expert enough to do this.
the BEST way is to go into iPhoto and find the "Preferences" panel and the set the "default editor" to Adobe Photoshop Elements. Now iPhot will hand off any photo in the library to Elements and when you save from inside Elements the phot goes back into the library.
The second method is to from inside iPhoto, select File->Export and export the file to the desktop or some folder. The start Elements and edit.
The first method is much better
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That would not work. Yu want to either tell iPhot to use PSE or do an export.
Your problem is you are still thinking this is a 1980's vintage PC. Just forget about keeping photos in folders. They are in a library.
The word "library" is accurate. How are books kept in a real public library? They are all in one big place kept in numbered order and you use a card catalog or now days a computer database search to find the book. That is how iPhoto works, it a data base search.