MS Enabler is a lightweight app (less than 2 megabytes) that lets you activate USB Mass storage mode with the memory card on your Android device. Only with the memory card, though. The app doesn't work with the device's internal memory.
The correct procedure for using UMS Enabler is the following: first you connect the Android device to a PC or a TV, and then you open the app and press the button to activate UMS mode. That's it. With just these two steps, you can activate UMS mode.
It's important to note, in order to use UMS Enable, you're Android needs to be rooted. Without root privileges, the app won't work correctly and actually, you shouldn't even install it.