Use the Security Assist Show region behavior.
Select the Login and register buttons, then go to the server behaviors panel (Window -> Server Behaviors) Click the plus button and select WebAssist -> Security Assist -> Show region. from the list of rules, select the "Not Logged Into <your users table>" (where <your users table> is the name of your users table) rule.
Add your logout button and the logged in message, then select them in design view, and add a second show region behavior, this time select the "Logged Into <your users table>" rule