So ya I have been fighting with this for a little bit now...each itunes and IOS update seems to change things and make things harder. Drag and drop just doesn't work and changing file names isn't working either...
What you need to do which makes it really really simple is this...
Pre-production. You will need an Mp3 version of your file. Look back over this thread if you need to figure out how to do that. Itunes will do it for you...right click...ect ect...
1. Open GarageBand.
2. Select Iphone Ringtone. (my version of garageband is 6.0.5) '11 Most versions of garage band have this function. at the very least you can just open a new project and skip to the next step.
3. Click on example ringtone or new project or whatever...just get it open.
4. Get rid of anything that is in the project. If you open example it will be a 30 second guitar thing or something...select it and delete.
5. Open Finder...drag copy of your Mp3 file into garage band.
6. Drag the yellow loop bar at the top of the arrange window to how ever long you want it to be.
7. click share
8. click export ringtone to itunes
9. go to itunes tone folder in itunes...drag to iphone, if you have manually manage selected...if you have sync selected cause you have a 1000000gig iphone for all your crap then just click sync.
on your iphone go to system preferences ... notifications ... app ... then there will be two places custom and downloaded "tones" will be. At the top is the alert tones section and at the top of that is the custom alert tones. Then scoll down and you will find the ringtones section. At the top of the ringtones section you will find the file that you just created in garageaband and moved onto your phone.
Hope that helps.