In Mendix 6.7, you can go to your navigation layout of choice, right click in the container where you want to have the sign out button, then: add widget -> Button -> Sign out button.
My solution, create a simple Java action with the following code:
Hope this helps :)
You can also use a custom button with a custom microflow with a java logout action for the account (in community commons module, where the account name is the openID variable for the javaaction). On this way you can include the logout into the navigation.