When you start bzadmin without any command line options other than callsign and hostname a simple curses-based user interface will be started (unless you built bzadmin without curses support). This interface is divided into three rectangles; the output window (which covers almost all of the terminal), the target window, and the input window.
The output window is where messages from the server will be shown.
The target window shows the name of the player that will receive your next message, or 'all' if it will be a public message. You can change target by using the left and right arrow keys.
The input window is where you type your messages. It also supports tab completion of commands and callsigns. You can clear the input window with the key combination Ctrl-U, and you can generate a /kick command for the current target with the F5 key (if the current target is a player). You can also generate a ban command for the current target with the F6 key, but this only works if you are an admin and the server has sent you that players IP address (as a response to a /playerlist command).
The curses user interface also has a simple menu system where you can edit the server variables (if you are an admin) and see a player list. If you are an admin you can also see the IP addresses in the player list, if you have sent a /playerlist command. The menu shows up when you hit the F2 key. It only covers the upper half of the screen, so you can still see what's going on in the game. You can navigate through the menu with the up and down arrow keys, and use the enter key to select submenus and other menu items. If you hit F2 again the command prompt will regain keyboard focus, but the menu will still be visible. If you hit F2 a third time the menu will go away.