Re-set up iMessages? I need help re-setting up iMessages.  I think I entered the wrong ID because now I can't get in.  It keeps telling me the Id or password is wrong, but I can't enter a new ID, just a new password.  How to re-set it up Please??  Thank you so much
EDIT: While the Messages app is active...

In the top left corner of your screen, click the bold Messages, then "Preferences", then the "Accounts" tab. 

On the iMessage account, hit the "Sign Out" button. Then it will allow you to enter in your password as well as Apple ID again.