First install a file explorer on your phone, I like ES.
You will have to create a new directory on your SD card to put your notifications under to use them so that they will show up in the stock ui.
Create a new folder called media, then in that folder create another called audio, then last create one called notifications. You just made the directory media/audio/notifications. Copy or move any .mp3 or audio file you want to use and put it under the last notifications sub folder.
Now the files will show up when you go to pick you notification sound for text messages or email ect., ect.
In the last notifications sub folder you can also create ringtones and alarms directories and put any file you want to use for those corresponding sounds also.