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The M-Audio KeyStudio is a MIDI controller keyboard - it doesn't make any sound, rather, it sends MIDI messages to instruct something else to make sound. Since you're running it into FL Studio, I presume you intend to control some software instrument, so you will need to load that instrument, route the MIDI messages to it, and enable monitoring. All of that will be done from within FL Studio, and should be documented in the manual.
All of that setup assumes that your keyboard is indeed correctly connected and sending MIDI messages. If FL Studio doesn't have any easy MIDI monitoring tool (I don't use it myself, so I don't know if it does), you can use another piece of software to monitor what comes out of the keyboard to ensure that it's working. On Windows I'm fond of MIDI-OX for this purpose.
