Did my research, worked around the issue.
Going to quote a review first:
"As I said above, this phone doesn't play mp3 ringtones. That is both true and false at the same time. This phone will play mp3's as ringtones ONLY if they are signed by T-Mobile, meaning if you like paying for your ringtones from your phone carrier then it will play them as ringtones. If you are like me and want to you your own mp3 collection as ringtones then this phone wont play them. It will NOT play them as mp3's, YOU WILL HAVE TO CONVERT THEM TO WMA files in order to them music files are ringtones. To some people that is a lot to ask for, but for me its just a small price to pay in order to get things the way I WANT."
(from Nokia 6133 - Features, Specs, and User Reviews - Mobiledia
So I took that tidbit and ran with it. Downloaded dbpoweramp (dBpoweramp Music Converter) and installed it. Updated it to the windows media player 10 spec (it asks when you try and convert to WMA format) and converted my MP3 to WMA. Multiple times until I got it to a size that was small enough to use without an SD card (on its way!) but large enough for decent quality.
After converting the file to WMA and uploading it to the phone, I was able to use the ringtone.
Hope this helps.