It depends on how you saved it to the memory and where it is. If you downloaded to your phone via your phone then there's no reason why it isn't listed in the ringtone list.
I also have a v3i and upload the ringtones via a lead and my pc. It's not clear where to save it to exactly when you use the phone's software with your PC and datalead. If when you scroll through the ringtones it isn't there listed, go into sounds in the media part and you should be able to find it there. If it's here you cant use it as a ringtone or transfer it to where it needs to be. You then need to go back to connecting the phone to your PC.
Open up Multi media Studio
Click on File Transfer Studio
Let the phone "talk" to the PC then find the ringtone on the pC on the left.
Then click on the folder with two dots under it in the phone panel on the right
Click on the audio file and rag the ringtone into it from the PC.
You should be able to find it on your phone once that's done.