On most receivers the display shows the format of the incoming signal, not how many speakers are currently being used. If the display shows left and right only it likely means the input signal is stereo.
There should be a button on the remote or front panel labeled something like 'surr'. Just press it until the desired mode is shown. If you select eg. ProLogic II, the display should then say PLII but the icons will still show left and right only.
I'm sure the manual describes the buttons on the remote in detail...