To make use of the admin functions, first you have to make sure that you are in the serveradmin group (any group will suffice as long as you have the right permissions to edit server- and usersettings). To edit a user simply click on the username and a dialog will pop up that allows you to mute, move, kick, poke etc. the user. To edit a channel simply click on the little blue arrow behind the channel, a dialog will come up that allows you to create sub-channels, edit the codec, password, description and just about anything you want.
This is also explained in our video tutorial here http://www.teamspeak.com/?page=tutorials&type=client#ts3_tutorial_4