Hmmm... I'm not sure we're understanding each other here. Let's see if I've got this right -
- You connected your phone to the computer and successfully created 3 individual new folders on the SD Card called - notifications, alarms & ringtones.
- The MP3 you want to use is also already on the SD Card.
- You now can't get the MP3 into the folder from the phone.
Ok, I
think what the issue is is that you're trying to move the MP3 around from the phone without the aid of a file explorer (one of these is not standard on the phone) downloaded. How are you seeing the MP3 on your phone? Just in the various select ringtone options and/or the music player? Anyway, the only way to move files around on the SD card from the phone is with a 3rd party app, such as Astro or File Explorer.
Have you tried moving the MP3 into the new "notifications" folder while you're still connected to the computer (I find this the easiest way)? You say it wont let you, right? This leads me to believe that you may be connected to the computer in charging only mode, although, if this is true, I don't know how you could create new folders.
Try this: On your phone, go to "Settings" > "Connect to PC" > and change the "Default connection type" to "Disc drive" (this is for the Hero, but it should be similar on your phone.)
Let's see if any of this helps...
Edit: If not, then maybe you should try an app such as Ringdroid out. I've never used it, so I can't help with it's functions, but I have seen it recommended often.