ok here goes. i have just tried this and it works fine but i will add a bit to it to hopefully help
Using your "My Files", in the directory of you sdcard, tab on the Menu key and select [Create folder], named it Media.
Tab Media folder, then Menu key and select [Create folder] again, name it Audio.
Tab Audio folder, then Menu key and select [Create folder] again, name it Alarms. Than another one Notifications and another one Ringtones.
1. open applications.
2. scroll through to find my files.
3. press menu button (bottom left of phone)
4. press create folder
5. in pop up box type media
6. now press media
to open folder
7. repeat step 4 and name the next folder audio
8. open audio
folder and repeat step 4 and name it notifications
9. do not open
notifications but repeat step 4 again to add afolder ringtones
and another folder alarms
10. now connect to pc using usb cable. on my computer it asks what i want to do and i select open folders. (you may have to look in my computer if not) you should see the folder media
which you created. open up
and you should find audio
. open this to reveal the three folders you created. open notification
now find your mp3 file on your computer, copy and then paste into notifications
11. disconnect cable, go into settings, press sound, press notification ringtones and it should be there
oops just noticed this is in the galaxy s section not s2.