Whats your operating system? My laptop & CPU won't recognize my phone as a Mass Storage Device either. It's because I have Vista on one and XP on the other. I tried it on my bro's Windows 7, and it worked. So if you have an OS other than Windows 7, I don't know if you're going to be able to enable mass storage mode, sorry!
This won't solve your problem with your phone, but you can always buy a USB SD card adapter if you're just trying to add stuff to your SD card. It's not as easy as plugging in your phone, but hey it works. Just unmount your card, plug in into your CPU with the adapter, and then put it back in your phone, you don't have to turn off your phone either of course.