Did you mount your device? It's been awhile since I've used an Android phone so bear with me if I'm using the terminology incorrectly.

The last Android phone I owned was the HTC EVO 4G (shortly after it came out) - for reference. To access the devices files back then, I'd have to swipe down from the top of the screen and select the kind of connection I wanted my device to have with the computer it was plugged into. There were three options but I only remember two. One was to charge with USB only, which automatically happened when I plugged the phone into the computer. Another option was "Mount as Disk" - I had to select this option to access any files on the phone from my computer. This [i]was[/i] a few versions of Android ago, so I don't know if it does it by default now or if you have to take similar steps.