The instant message check box at the bottom of the View Contacts page is to transfer a contact using the button below it. If this box is checked, and if the contact group the contact is being transfered to has an instant message, the contact will be sent the instant message. Otherwise, the contact will not be sent an instant message when being transfered.
To edit the default message go to Edit Messages and at the bottom edit:
Confirm Your Email (Double-Optin) Email
Also, when editing a contact group there is the option to add/edit a specific message for that group.
When using double optin, the instant message is sent when a person clicks the confirmation link in the email that is sent.