Did you remember to unzip the file? The file you d/led from bittorrent is the .iso that has been compressed using open source 7zip. You need to go here http://www.7-zip.org/
, d/l and install, then unzip the .iso.7z file, and since you're using nero, double click on the .iso file that is extracted. Just like a .rar or .zip file, gotta extract it first
Hope that helps, if you have more questions, feel free to ask. Also, I would suggest to ensure that you close the disk session out when asked about the burning details from Nero.