I have the Casio WK-3500 so I'll try to answer your questions. Also the arranger forum as 3351 stated would be the best place on the Zone to post your questions about the Casio's and other arrangers.
Can you select any sounds other than piano using the tone button (white round button at right of screen)? You may want to do a factory reset. Press the transpose/function button at top left of screen. Then press the down cursor (3)times. The screen should say-InitMix?- Press the right cursor (2) times and the screen should read-InitSys?- Now press the yes button and continue to confirm the initial reset.
Regarding your memory for songs.. The board will hold around 10,000 notes for songs. If you have several songs already in memory you may want to save some to smart media and open up more space. You can tell if you're low on memory by trying to record a song. If your memory is full the board will automatically stop recording. Here's how you delete a song. Press the song button. Then select the song you want to delete using the + and - keys. Now press the down cursor (2) times to display the song delete screen. Press (yes) and the board will ask if you're (sure). Press (yes) again to delete the song. Hope this helped.
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.