I would like to create my own ringtone for my side kick slide

Is this possible at all ? I have typed in ceate ringtone in a search engine,and got a few good websites on how to do this. I went on the websites,I uploaded the songs I wanted. And had them sent to my sidekick slide in a mms mesage and a regular text mesages. They came to my phone,but I couldnt set them as my ringtones. A few other site gave me the option to create the ringtone on the site,then go on the site on my phone and try to download the ringtone that way. It still wouldnt let me get the ringtone into the imported box,which is the box that the other ringtones I downloaded and paid for go into. The ringtone has to be in the imported box to set as a ringtone. So! I want to know if there is anyway at to create my own personal ringtone and use it on my sidekick slide ???