As explained in Microsoft KB article 2383602 (http://support.microsoft.com/kb/2383602/en-US), attachments are opened in read-only mode with newer versions of Outlook.
Sometimes, it's useful to be able to edit emails or associated attachments.
To make this possible, we provide a green button in the Quick Access toolbar, which is the toolbar appearing in the top left corner when you open a message. To make the message editable, click on that green button. Once clicked, both the message itself and any attachments will be writeable.
This screen shot shows the location of the green button: