When trying to edit an action in a database, the design pane showed "This object's data may not be modified.". He could not work out how he could edit the action.

Turns out this is a Shared Action.

To edit this action you have to go to

    Resources>Other>Shared Actions

and the action will be there.

This will mean that if you edit the action it might be used in many views or forms and so you should test any changes in all locations and circumstances.