It was so easy it foiled me for two days...I was trying my hardest to get into the filesystem but sometimes the simplest solution is the best.
1. Plug in the supplied usb cable, select "pc suite" on your phone and let your computer install the drivers and the pc suite software (it will prompt to download it if you don't have a disk). I took this opportunity to dump my contacts (in a .csv file) into the phone using PC Suite which is a great little program IMO.
2. Shut down the PC Suite program and unplug your phone and reconnect it. This time select "Mass Storage" on the phone.
3. Your computer should now recognize it just like a usb flash drive. Open the folder (mine only had one folder there) and drag or copy your ringtone (mp3) into it. Now click cancel on the phone and disconnect the usb cable.
4. Now in the phone go to settings>my stuff>sounds, select the tone you just put in and click the settings tab (top left) and select "use as > whatever"