Motorola Droid Razr :: Installing The Custom Ringtone?

I'm having trouble installing a custum ringtone. I've downloaded it as an MP3 to the My Music folder and also copied it to a new folder that I've called "music" on my external memory card. So I'm able to play the tune from both locations but I can't find any menu option that allows me to set it as the ringtone. When I hold down the song in My Music a menu comes up that gives a number of options starting with Play and ending with Copy to DLNA server and Delete. The Mortorla phone tech person said that between the last two on his phone was a Set as ringtone option but it does not exist on mine. He offered to reset my phone to the original factory settings. But I would loose too much stuff so that is not an option.how I get to set it as a my new ringtone?