Editing with External Editors

Heya folks, a quick query as i suspect ive probably missed something, i have Pixelmator installed as a cheaper image editor with much of the functionality of Photoshop, and i want to use it as my external editor. How do i go about doing that? I go to Images > Edit With> but the option for external editor is greyed out.
Any ideas?


from page 350 of the user's manual:
To set an external editor in Aperture:
1 Choose Aperture > Preferences, or press Command-Comma (,), then click Export.
2 Click the Choose button below the External Image Editor field, navigate to an
application in the Select Application dialog, then click Select.
The application’s name appears in the External Image Editor field.
3 Choose the appropriate 16-bit file format (TIFF or PSD) from the External Editor File
Format pop-up menu.
This is the file format Aperture uses when opening files in external editors.