How do you use mp3s as ringtones on a Sony Ericsson w760a Walkman cell phone?

[american_idol_fan15]

I downloaded Media Manager, I added an mp3 to my phone, yet it wouldn't let me use it as a ringtone only a message alert this really pissed me off because the idiot working in the electronics department at Wal-Mart told me I would be able to do that with the phone so please tell me step by step instructions in order to do this. It would be appreciated.