DO NOT connect the USB cable until the instructions below say so.
1. On the phone go to Settings > Wireless and network > USB utilities
2. On the screen that says, "USB mass storage," click the button "Connect storage to PC."
3. When you see the pop-up that says, "USB utilities," THEN connect the USB cable from phone to computer.
4. Then you see the Android, and it says, "USB connected." Click "Connect USB storage." NOW you will finally be connected, like any normal USB device. On a Mac, you will see the Galaxy S2 mounted on the desktop. On Windows, it probably appears like any other USB device in "My Computer."
If you don't follow the steps exactly, you can't see the files.
This is the _stupidest_ process for connecting any USB device I have EVER seen!