My new Galaxy SII phone arrived yesterday and I have not yet learned all the ins and outs. I've had an iPhone for three years so Android is new to me. However, I did find an easy way to do this if you have not yet learned how to add a folder or work in the root or even know what the root is...yet!
Install Kies Air (it's in your apps) to connect to your wireless PC or laptop. Then open the app and follow instructions to go to the URL given. Once there, on the left, click on Ringtones. You'll see all the ringtones that come with the phone. You will also find a place where you can choose to "upload." I had saved the MP3 file to my desktop, so I went there, selected it, and clicked install. I then set it as the default ringtone while still in Kies Air.
I then went to my phone and checked the ringtone list, it was there, and was set as the default.
WOW...I could never do this on the iPhone. How cool is this system?
Mary