Hi,just bought this camera and tried access it as mass storage device.

Goto System Preferences and select USB.
Then change to Devices page.
Connect the camera.
It's shown as a mass storage then.
Select device Pentax and click on Settings.
"Single LUN" "No Initial Reset" and "Simple SCSI" must be selected, other checkboxes deselected.
Click on "Use & Set as Default".

Now it should work...