I often hear from Apple Aperture users who are having trouble opening thier pictures in Photoshop. In 99 out of 100 cases, the problem is a simple one. They can’t access the menu command, Open with External Editor and it doesn’t appear in the contextual menus and/or is grayed out in the Images menu.
The reason for this is that they have yet to specify an external editor.
In order to get this working, go to Aperture > Preferences and a dialog box will open. About two thirds of the way down you will see a space that says External Editor.
Underneath that is a button that says set. Click the set button and navigate to your external editor. It will probably be located in your Applications folder on your main hard disk. Once you find it and select the program, Aperture will automatically open Photoshop every time you select Images Open With External Editor.