A number of people have had problems installing ringtones that they didn't buy from Rogers on their phones. I have found 2 ways that work on my Nokia 5300:

1) Use wav files. I assembled the ringtone I wanted in Nero Wave editor (it came with my burner - I'm sure there are free programs now that will do more) and saved it as a wav file with 8000 Hz sample rate and 8 bit depth. The phone allowed me to select "Use tone" and set it as a ringtone or msg. alert.

2) Create a theme with Zedge Theme Maker and include the MP3 you want as part of the theme. When I installed the theme the MP3 was automatically set as the ringtone.

I would recommend using some kind of sound editing software to make a clip several seconds long that will sound OK when played several times. If you use whole songs the theme file will be larger than needed.