I assume you use the free addition in combination with the appcloud module.
In that case MxAdmin isn't available, but you can login using your Mendix account.
In the app. via the menu item "Accounts" local accounts can be added/managed.
Or accounts using the Mendix account credentials can be added via launchpad: http://launchpad.mendix.com/
The cloud uses another database then when you run the app locally. Try login in with MxAdmin account and you'll see that there are no users in the cloud.