Raw photo downloads failing

Help! Read loads of stuff on this but still can't get my raw files to download on to lightroom. I'm a total rookie having just started with lightroom so any help is much appreciated. My raw files are supported by lightroom: CR2 from my Canon 1Ds Mark II. Do I need to dowload the Camera Raw plug in to make it work? Files seem to be downloading but then after a few minutes it just fails.