Why can't some one explain this problem clearly even from apple guys. ClaraMGR i know your problem clearly.
Latest I tunes can't convert your mp3 to ACC even if you change the extension from mp3 to ACC it wont allow you to drop in the tones.
This is not easy task, am computer tech guy i know how things works but i always ended up fighting with user interface with iTunes its very complected for general users. I spend almost a half day to figure out whats wrong with it. finally made my own crack. Am still not sure if this is right way but 100 percent it works not matter what your format is.
I imagine you Followed all this steps.
1. iTunes preffs > General. Click the Import Setings button and set it to AAC.
2. Make sure iTunes prefs > General has Tones ticked
3. This is the key point you need some converter mp3 to AAC i used soundforge open a mp3 in soundforge got to file save as AAC/MP4
4. rename the saved AAC to m4r. IF you don't see the extension type in to youtube unhide extension in windows.
5. Drag and drop the file in to itunes tone and go enable the itunes tone sync and click the sync it will convert the file to ringtone and puts in to your itunes tone folder automatically you done. Thats all i hope this will help some one.
6. got to iphone>general>sounds you will see your file on the top
Why this is not a one click button 😟, Would be a lot easier if drag the file and right click convert to tone job done right. Why would some one make this so complected to spend days to figure out this is not a some thing we are achieving this is just a insane why would we spend hours of our valuable time to make ringtone this way. this isn't a one thing even i don't know how to grab my own photos from my iPhone its like am running circles around again and gain got nothing. Itunes is not user friendly, I really don't know how itunes software development user testing tools were implemented its pathetic. Even when i open to buy a apps thats insane shocking i never open that again, i dont find any proper apps, none of them is user friendly its so confusing that freaks me out spending my time on it, Am big fan of apple but am trying to convince my self with itunes but nooooooooooooooo its not easy and i got no time for thisssssssssssssssssssssssssssss