Curves in CS5

Hello,
I have, fairly recently, upgraded from Elements to CS5. I have run into a slight problem which is baffling me. Whatever route I take to access Curves, I don't get the full facilities. For example, there are no sliders and I get a "No Entry" sign when I hover over the Target Adjustment Tool button. I would appreciate some advice.
Thanks!

Are you working in RGB 8 bits/Channel? that might help
Try adding an "Adjustment Layer" or just go to Image > Adjustments > Curves