There's a problem in Adobe Acrobat 5.0.5 for Mac OS X where, after the selection of the 'Edit Object' contextual menu item, Acrobat prompts with an error saying the editing program could not be launched. This is a known issue, which Adobe describes (along with inconvenient workarounds... including running Acrobat and the editing application in Classic mode!) in its Knowledge Base. Though the editing application (for example, Adobe Photoshop 7.0 or Adobe Illustrator 10) opens, the changes you make to the object aren't automatically updated in the PDF file.
To fix the image editor bug, I opened the Photoshop package and found the alias directly inside (at the top level of the package folder) that supposedly points to the executable. If you copy that alias outside the package, you can then select it (rather than the .app package) from within the Acrobat preferences for image editors. This "tricks" Acrobat into working with Photoshop -- no error messages and when you save changes in PS, it actually updates! I also did something fairly similar when getting Acrobat to work with Illustrator.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030226142356661