Welcome to AC..
By default... its set to Charge Only when plugged into the computer. If you want it to show up as a drive... turn on USB Debugging from your phone... Menu.. Settings.. Applications.. Development.. USB Debugging "ON". From the phones notification/pull down bar... switch to Mount as Disc Drive or Charge Only.
Stop by the Bionic Forums if you need help or have any questions. Another good place to start is the General Help & How To and Tips & Tricks threads found in my signature.