using keis and trying to use your phone...
as a mass storage device are 2 completely different applications. you don't have to be running one of these apps to run the other. when i first got my galaxy, i plugged the usb into the phone and my pc expecting my computer to correctly read my phone's memory and memory card-but instead got the camera reading box you reffered to. i got the correct instructions for using the galaxy s2 as a mass storage device directly from samsung.com and choosing the 'support' tab at the top of the page.
this is the link... http://www.samsung.com/us/support/SupportOwnersFAQPopup.do?faq_id=FAQ00034625&fm_seq=34793
i do remember the first couple of times i used the instructions that if i plugged the usb cord into my PC without pressing the correct instructions on the phone first that the camera reader icon popped up on my PC. i had to close the camera reader window, unplug the usb cable from my PC so that it didn't read it anymore and start over again. if you're still having issues, maybe your local sprint store will show you how to do it in person. mine offered to show me until i figured out how to do it myself.