When your phone is attached to the computer, expand it and you should see a ring tones folder under your phone. Click and drag the ringtones to that folder. Sync and they should be on the phone now.
You can also check to be sure your syncing all ringtones too.
I have no problem with my iTunes account making a ring tone. I simply go to my playlist, right click on the selected song, then select get info/options and check the two boxes for up to 30 seconds of play time. Next I go back to the song and create aac version. Then I right click on the aac version and select show in windows explorer.
Now here's the issue, on my in my account, the "name" shows the song with a .m4a extension that I change to a .m4r extension. Then I simply add the file to my tones library and I'm good to go - but on my wifes account, I do exactly the same thing but when I go to the file where it shows the name of the song (same as above), there is no extension at the end of the songs name to change (as an example: stairway to heaven.m4a shows on my folder and on hers it only stairway to heaven).
I can make ring tones easy, but I cannot add her song to her library on iTunes.
Hopefully this makes sense
I believe what's happening here is that while you are creating a smaller file, renaming the file doesn't make it a viable ringtone. Resizing to 30 seconds isn't sufficient. If Apple allowed this method, it would usurp their ability to sell ringtones. People could make ringtones from any song in their music collection. That's certainly possible with a number of apps, but not from within iTunes.
Your best bet would be to invest in a great ringtone app. My favorite is Ringtonium. You'll be able to create a ringtone on your iPhone. This gets synced back through iTunes where you'll drag it to your desktop and drag it back into iTunes. It will then be recognized as a valid Tone. This tone can be shared with other iTunes users as well.
I realize this answer may be a little late but, have you ever tried iRinger? It's a computer program that will let you select the section of the song you want, set the length of the ring tone, increase the volume, convert it to the proper format and will even place the ringtone into the proper iTunes folder automatically. The best part is that it is free.