Unfortunately a mp3 will be considered as a single music track. So therefore if it's longer than 80 minutes (current standard audio cd length, used to be 74 minutes) it will not burn.
I recommend converting the mp3 to wave file (winamp diskwriter plugin can do this for you) and edit the wave file to a more reasonable length.
If the mp3 is sorta like 81 minutes in length, you could also try overburning and/or use 99 minutes cd-r's. You can enable this in Nero using the preferences setup in the menu.