I can tell you what worked for me. I can't make any promises it will work for you:
First you need the modem driver. Get that here. (Note: This is Windows software. I haven't got a Mac, so if you're using one of them you will need to find a version of the drivers for it. I wouldn't know where to look.) Install that driver. Without it, you cannot communicate with the phone.
Then get the latest BITPIM. As of this writing, that's 1.0.6. You can find it at the Bitpim site. Install that.
Connect your phone to your computer using the USB charging cable that came with it.
The current version of Bitpim does not support this phone directly, so you have to play a game. That game is called "modify the filesystem directly." This is a dangerous game. A misstep here could brick your phone. BE CAREFUL.
If the phone goes into Music Sync mode, cancel out of that. Set the phone's USB mode to modem: Menu, then Settings and Tools, then USB Mode. Set it to Modem Mode.
Bitpim should detect your phone at this point, calling it 'Other CDMA Phone on COMx' where the 'x' is some number.
In Bitpim, click 'View' and then 'View Filesystem'. A filing cabinet icon should appear in the left hand tree. Click on that and a closed folder should appear just to the right of this tree. Click on the '+' character to the left of this folder. This is called 'expanding' the folder. You should see a 'brew' folder; expand that. Under it, look for the 'mod' folder and expand it. Under it, look for the '10889' folder and expand it. Now you should see a 'ringtones' folder.
You can drag .mp3 files to this folder from other folders on your machine. Notoe that it takes several second to transfer a file, so if you have a lot of them to transfer, expect to wait.
When your files are transfered, exit Bitpim, shut the phone off, and restart it. The ringtones should appear.
I'm going to say this again: IF YOU ALTER THE WRONG FILE IN THE FILESYSTEM VIEW YOU CAN DAMAGE YOUR PHONE, PERHAPS EVEN RENDER IT UNABLE TO BOOT. EVEN IF YOU HAVE INSURANCE A CARRIER CAN SOMETIMES DETECT SUCH 'UNAUTHORIZED' ACTIVITIES AND MAY REFUSE TO HONOR THE POLICY. I recommend you go slowly and carefully, and if something happens that doesn't match these instructions precisely, search around for someone who knows more about these phones than I do. I took a certain amount of risk doing this, it worked for me, but not everyone is that risk tolerant.