this is how you transfer files between your phone and PC...

i have the sprint galaxy s2 epic 4g so these steps should work for you ...
plug the USB cable into your phone.
click 'settings' on your phone then click on the 'wireless and network' option
scroll down to and click on 'USB utilities'
a box should pop up on your phone saying 'connect storage to PC' so click on it
now plug the other end of the USB into your PC
a box should pop up on your phone saying 'connect USB storage' so click on it
2 boxes should pop up on your PC that now allow you to see your phone's files. On my PC
'k' is my phone's memory and 'L' is my phones memory card. When you're done moving files between
your phone and PC, just click on the 'diconnect storage from PC' box on your phone and you can
now diconnect the usb cable from the phone and PC. Hope this helps!