Bro, thank you. It really work.
For those dun understand, u can go to My file> Media folder> Audio.
Then create 2 separate folders, name it as Ringtones & notifications respectively
And put your mp3 file into the folders.

Then the mp3 will be appear in your ringtone list
You may check it here
Setting>sound and display> voice call ringtone/notification ringtone
To remove the mps from you ringtone list, just delete the mp3 file from your Ringtones/notifications folder.